using System;
using System.Collections.Generic;
using System.Numerics;
using System.Runtime.InteropServices;
using AcDream.Core.Meshing;
using AcDream.Core.Rendering;
using AcDream.Core.Terrain;
using AcDream.Core.World;
using DatReaderWriter.Enums;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering.Wb;

/// <summary>
/// Draws entities using WB's <see cref="ObjectRenderData"/> (a single global
/// VAO/VBO/IBO under modern rendering) with acdream's <see cref="TextureCache"/>
/// for bindless texture resolution and <see cref="AcSurfaceMetadataTable"/> for
/// translucency classification.
///
/// <para>
/// <b>Atlas-tier</b> entities (<c>ServerGuid == 0</c>): mesh data comes from WB's
/// <see cref="ObjectMeshManager"/> via <see cref="WbMeshAdapter.TryGetRenderData"/>.
/// Textures resolve through the bindless-suffixed
/// <see cref="TextureCache.GetOrUploadBindless"/> variants, returning 64-bit
/// resident handles stored in the per-group SSBO.
/// </para>
///
/// <para>
/// <b>Per-instance-tier</b> entities (<c>ServerGuid != 0</c>): mesh data also from
/// WB, but textures resolve through
/// <see cref="TextureCache.GetOrUploadWithPaletteOverrideBindless"/> with palette
/// and surface overrides applied. <see cref="AnimatedEntityState"/> is currently
/// unused at draw time — GameWindow's spawn path already bakes AnimPartChanges +
/// GfxObjDegradeResolver (Issue #47 close-detail mesh) into <c>MeshRefs</c>.
/// </para>
///
/// <para>
/// <b>GL strategy (N.5 — mandatory):</b> <c>glMultiDrawElementsIndirect</c> with SSBOs
/// and <c>GL_ARB_bindless_texture</c> + <c>GL_ARB_shader_draw_parameters</c>.
/// All visible (entity, batch) pairs are bucketed by <see cref="GroupKey"/>;
/// each group becomes one <c>DrawElementsIndirectCommand</c>. Three GPU buffers
/// are uploaded per frame: instance matrices (SSBO binding 0), per-group batch
/// metadata/texture handles (SSBO binding 1), and the indirect draw commands.
/// Two <c>glMultiDrawElementsIndirect</c> calls cover the opaque and transparent
/// passes respectively — one GL call per pass regardless of group count.
/// </para>
///
/// <para>
/// <b>Shader:</b> <c>mesh_modern</c> (bindless + <c>gl_DrawIDARB</c> /
/// <c>gl_BaseInstanceARB</c>). Missing bindless/draw-parameters throws
/// <see cref="NotSupportedException"/> at startup — there is no legacy fallback.
/// </para>
///
/// <para>
/// <b>Modern rendering assumption:</b> WB's <c>_useModernRendering</c> path (GL
/// 4.3 + bindless) puts every mesh in a single shared VAO/VBO/IBO and uses
/// <c>FirstIndex</c> + <c>BaseVertex</c> per batch. The dispatcher honors those
/// offsets inside each <c>DrawElementsIndirectCommand</c> via
/// <c>glMultiDrawElementsIndirect</c>.
/// </para>
/// </summary>
public sealed unsafe class WbDrawDispatcher : IDisposable
{
    /// <summary>
    /// Which subset of entities to walk in a single Draw call.
    ///
    /// Phase U.1 (2026-05-30): the indoor/outdoor two-pipe split (IndoorPass /
    /// OutdoorScenery / BuildingShells / LiveDynamic) was deleted along with the
    /// inside-out render machinery. <see cref="All"/> is the sole remaining
    /// member; the unified retail-faithful pass (Phase U) draws every entity in
    /// one path. The <c>set:</c> parameter is retained on the Draw overloads so
    /// the unified pass can re-introduce partitioning later without re-threading
    /// the call sites.
    /// </summary>
    public enum EntitySet
    {
        /// <summary>Every entity walked, gated only by the existing
        /// <c>ParentCellId ∈ visibleCellIds</c> filter.</summary>
        All,
    }

    private readonly GL _gl;
    private readonly Shader _shader;
    private readonly TextureCache _textures;
    private readonly WbMeshAdapter _meshAdapter;
    private readonly EntitySpawnAdapter _entitySpawnAdapter;

    private readonly BindlessSupport _bindless;

    public readonly record struct DrawStats(
        EntitySet Set,
        int EntitiesWalked,
        int MeshRefs,
        int Instances,
        int Draws,
        int CullRuns,
        int OpaqueDraws,
        int TransparentDraws,
        long Triangles);

    public DrawStats LastDrawStats { get; private set; }

    // Tier 1 cache (#53): per-entity classification results for static
    // entities (those NOT in GameWindow._animatedEntities). Wired here in
    // Task 7 for plumbing only — Tasks 9-10 wire the per-entity
    // miss-populate / hit-fast-path through the loop.
    private readonly EntityClassificationCache _cache;

    // ACDREAM_DISABLE_TIER1_CACHE=1 A/B diagnostic — forces every static
    // entity through the slow path. Read once in ctor.
    private readonly bool _tier1CacheDisabled =
        string.Equals(Environment.GetEnvironmentVariable("ACDREAM_DISABLE_TIER1_CACHE"), "1", StringComparison.Ordinal);

    /// <summary>
    /// A.5 T22.5: gate for GL_SAMPLE_ALPHA_TO_COVERAGE around the opaque pass.
    /// Default true matches T20 behavior. Set false for Low/Medium presets that
    /// have MsaaSamples=0 (A2C is a no-op without MSAA, but turning it off
    /// avoids the unnecessary GL state thrash and is cleaner diagnostics).
    /// Can be toggled mid-session via <see cref="GameWindow.ReapplyQualityPreset"/>.
    /// </summary>
    public bool AlphaToCoverage { get; set; } = true;

    // SSBO buffer ids
    private uint _instanceSsbo;
    private uint _batchSsbo;
    private uint _indirectBuffer;

    // Phase U.3: per-instance clip-slot SSBO (binding=3), parallel to
    // _instanceSsbo. One uint per instance selecting its CellClip slot. In U.3
    // this is ALL ZEROS (every instance → slot 0 → no-clip), so the render is
    // identical to pre-U.3. U.4 populates real slot indices.
    private uint _clipSlotSsbo;
    private uint[] _clipSlotData = new uint[256];

    // Phase U.3: the SHARED per-cell clip-region SSBO (binding=2), owned by the
    // GameWindow-level ClipFrame and handed to us via SetClipRegionSsbo. When 0
    // (not yet wired), we bind our OWN fallback no-clip region buffer below so the
    // shader never reads an unbound SSBO. The fallback holds exactly slot 0
    // (count 0 = pass-all), matching ClipFrame.NoClip's slot 0.
    private uint _sharedClipRegionSsbo;
    private uint _fallbackClipRegionSsbo;

    // Phase U.4: per-frame clip-slot routing handed in via SetClipRouting before
    // each Draw. When _clipRoutingActive is false (the U.3 path / outdoor root /
    // no portal frame), every instance maps to slot 0 (no-clip) and no instance is
    // culled — identical to U.3. When active, each instance's slot is resolved by
    // ResolveEntitySlot per the U.4 policy (live-dynamic unclipped; cell statics to
    // their cell slot; outdoor scenery to the OutsideView slot; non-visible culled).
    private bool _clipRoutingActive;
    private IReadOnlyDictionary<uint, int>? _cellIdToSlot;
    private int _outdoorSlot;
    private bool _outdoorVisible;

    // Phase U.4: the clip slot of the entity currently being classified in Draw's
    // per-entity loop. Set once per entity (before ClassifyBatches / ApplyCacheHit),
    // read by the two matrix-append sites (AppendInstanceToGroup + ClassifyBatches)
    // so every group's Slots[] stays in lockstep with its Matrices[]. Defaults to 0
    // (no-clip) on the U.3 / outdoor path.
    private uint _currentEntitySlot;

    // Phase U.4: true when the current entity resolved to the CULL sentinel
    // (cell not visible, or outdoor stab while no outdoors is visible). Persisted
    // across the entity's tuples; the per-tuple body skips all instance emission.
    private bool _currentEntityCulled;

    // Per-frame scratch arrays — Tasks 9-10 fully wire these.
    private float[] _instanceData = new float[256 * 16];      // mat4 floats per instance
    private BatchData[] _batchData = new BatchData[256];
    private DrawElementsIndirectCommand[] _indirectCommands = new DrawElementsIndirectCommand[256];
    private CullMode[] _drawCullModes = new CullMode[256];

    private int _opaqueDrawCount;
    private int _transparentDrawCount;
    private int _transparentByteOffset;

    // std430 layout: ulong TextureHandle (uvec2) at offset 0, uint TextureLayer
    // at offset 8, uint Flags at offset 12. Total 16 bytes.
    // Pack=8 (not 4) because std430's uvec2 requires 8-byte alignment — Pack=4
    // works today by accident (TextureHandle is the first field, so offset 0 is
    // always 8-byte aligned), but adding a 4-byte field before TextureHandle
    // without bumping Pack would silently misalign the GPU struct.
    [StructLayout(LayoutKind.Sequential, Pack = 8)]
    private struct BatchData
    {
        public ulong TextureHandle;   // bindless handle (uvec2 in GLSL)
        public uint  TextureLayer;
        public uint  Flags;
    }

    // Per-frame scratch — reused across frames to avoid per-frame allocation.
    private readonly Dictionary<GroupKey, InstanceGroup> _groups = new();
    private readonly List<InstanceGroup> _opaqueDraws = new();
    private readonly List<InstanceGroup> _translucentDraws = new();
    // A.5 T26 follow-up (Bug B): WalkEntities populates this scratch list
    // instead of allocating a fresh List<(WorldEntity, int)> per frame. At
    // ~10K entities × ~3 mesh refs = ~30K tuples × 16 bytes = ~480 KB / frame
    // of GC pressure on the render thread under the original T17 shape.
    private readonly List<(WorldEntity Entity, int MeshRefIndex, uint LandblockId)> _walkScratch = new();

    // Tier 1 cache (#53) — per-entity classification collector. Reused across
    // frames; cleared at flush time when the per-entity loop crosses an entity
    // boundary in _walkScratch (and once more at end-of-loop for the last
    // entity). _walkScratch is in entity-order, so all MeshRefs of one entity
    // are contiguous — accumulate them all before flushing one Populate call.
    // Animated entities skip this scratch entirely (collector = null).
    private readonly List<CachedBatch> _populateScratch = new();

    // Per-entity-cull AABB radius. Conservative — covers most entities; large
    // outliers (long banners, tall columns) are still landblock-culled.
    private const float PerEntityCullRadius = 5.0f;

    private bool _disposed;

    /// <summary>
    /// Per-cell-entity last-log frame number for rate-limiting the
    /// [indoor-walk] / [indoor-lookup] / [indoor-xform] / [indoor-cull]
    /// probes. Defaults to 30 frames at 30Hz = 1 sec.
    /// </summary>
    private readonly Dictionary<ulong, int> _lastIndoorProbeFrame = new();
    private int _indoorProbeFrameCounter;
    private const int IndoorProbeRateLimitFrames = 30;

    /// <summary>
    /// Returns true at most once per <see cref="IndoorProbeRateLimitFrames"/>
    /// frames per cellId. Caller must already have checked that an indoor
    /// probe flag is enabled.
    /// </summary>
    private bool ShouldEmitIndoorProbe(ulong cellId)
    {
        if (!_lastIndoorProbeFrame.TryGetValue(cellId, out int last)
            || _indoorProbeFrameCounter - last >= IndoorProbeRateLimitFrames)
        {
            _lastIndoorProbeFrame[cellId] = _indoorProbeFrameCounter;
            return true;
        }
        return false;
    }

    // Diagnostic counters logged once per ~5s under ACDREAM_WB_DIAG=1.
    private int _entitiesSeen;
    private int _entitiesDrawn;
    private int _meshesMissing;
    private int _drawsIssued;
    private int _instancesIssued;
    private long _lastLogTick;

    // CPU + GPU timing for [WB-DIAG] under ACDREAM_WB_DIAG=1.
    private readonly System.Diagnostics.Stopwatch _cpuStopwatch = new();
    private readonly long[] _cpuSamples = new long[256];   // microseconds
    private int _cpuSampleCursor;
    // GPU timing uses a ring of 3 query-pair slots so the read of frame N-3's
    // result lands when the GPU has finished (~50ms after issue on a typical
    // 60fps frame). Ring of 3 is the vendor-neutral choice: NVIDIA drivers with
    // triple-buffering+vsync can queue ~3 frames ahead, AMD typically 1-2,
    // Intel iGPUs vary. ResultAvailable is the safety guard if the GPU is
    // still working when we try to read.
    private const int GpuQueryRingDepth = 3;
    private readonly uint[] _gpuQueryOpaque      = new uint[GpuQueryRingDepth];
    private readonly uint[] _gpuQueryTransparent = new uint[GpuQueryRingDepth];
    private int _gpuQueryFrameIndex;
    private readonly long[] _gpuSamples = new long[256];   // microseconds
    private int _gpuSampleCursor;
    private bool _gpuQueriesInitialized;

    // Constructor accessibility is internal because EntityClassificationCache
    // is internal — a public ctor with an internal-typed parameter would be
    // an inconsistent-accessibility error. The dispatcher is constructed
    // exclusively from GameWindow (same assembly), so internal is fine.
    internal WbDrawDispatcher(
        GL gl,
        Shader shader,
        TextureCache textures,
        WbMeshAdapter meshAdapter,
        EntitySpawnAdapter entitySpawnAdapter,
        BindlessSupport bindless,
        EntityClassificationCache classificationCache)
    {
        ArgumentNullException.ThrowIfNull(gl);
        ArgumentNullException.ThrowIfNull(shader);
        ArgumentNullException.ThrowIfNull(textures);
        ArgumentNullException.ThrowIfNull(meshAdapter);
        ArgumentNullException.ThrowIfNull(entitySpawnAdapter);
        ArgumentNullException.ThrowIfNull(classificationCache);

        _gl = gl;
        _shader = shader;
        _textures = textures;
        _meshAdapter = meshAdapter;
        _entitySpawnAdapter = entitySpawnAdapter;
        _cache = classificationCache;

        _bindless = bindless ?? throw new ArgumentNullException(nameof(bindless));
        _instanceSsbo   = _gl.GenBuffer();
        _batchSsbo      = _gl.GenBuffer();
        _indirectBuffer = _gl.GenBuffer();
        _clipSlotSsbo   = _gl.GenBuffer();          // Phase U.3 binding=3
    }

    /// <summary>
    /// Phase U.3: hand the dispatcher the SHARED per-cell clip-region SSBO
    /// (binding=2) that <see cref="ClipFrame.UploadShared"/> created. The
    /// dispatcher re-binds it to binding=2 immediately before each MDI so a
    /// consumer that touched binding=2 in between can't leave it pointing
    /// elsewhere. Pass 0 to fall back to the internal no-clip region buffer.
    /// </summary>
    public void SetClipRegionSsbo(uint sharedClipRegionSsbo)
        => _sharedClipRegionSsbo = sharedClipRegionSsbo;

    /// <summary>
    /// Phase U.4: install the per-frame clip-slot routing for an INDOOR root.
    /// Call once per frame BEFORE <see cref="Draw"/> when the camera's root cell is
    /// non-null; the next <see cref="Draw"/> resolves each instance's binding=3
    /// clip slot via the U.4 policy (live-dynamic unclipped, cell statics to their
    /// cell slot, outdoor scenery to the OutsideView slot, non-visible culled).
    /// Pair with <see cref="ClearClipRouting"/> on outdoor-root frames so the
    /// dispatcher reverts to the U.3 no-clip-everything behavior.
    /// </summary>
    /// <param name="cellIdToSlot">cellId → CellClip slot. A cell absent from the map
    /// is NOT visible → its cell-static instances are culled.</param>
    /// <param name="outdoorSlot">Slot for outdoor scenery / building shells while
    /// indoors (the OutsideView slot, or 0 for no-clip over-include).</param>
    /// <param name="outdoorVisible">False ⇒ cull outdoor scenery / shells this frame
    /// (the OutsideView is empty).</param>
    public void SetClipRouting(IReadOnlyDictionary<uint, int> cellIdToSlot, int outdoorSlot, bool outdoorVisible)
    {
        ArgumentNullException.ThrowIfNull(cellIdToSlot);
        _clipRoutingActive = true;
        _cellIdToSlot = cellIdToSlot;
        _outdoorSlot = outdoorSlot;
        _outdoorVisible = outdoorVisible;
    }

    /// <summary>
    /// Phase U.4: revert to U.3 behavior — every instance maps to slot 0 (no-clip),
    /// nothing is culled by clip routing. Call on outdoor-root frames (camera
    /// outdoors) and any frame without a portal-visibility result.
    /// </summary>
    public void ClearClipRouting()
    {
        _clipRoutingActive = false;
        _cellIdToSlot = null;
        _outdoorSlot = 0;
        _outdoorVisible = false;
    }

    // Phase U.4 CULL sentinel returned by ResolveEntitySlot: the entity's instances
    // are dropped entirely (not emitted into the binding=0 instance buffer NOR the
    // binding=3 slot buffer), matching the existing frustum / visible-cell cull.
    // Internal (not private) so the clip-slot unit tests can assert against it
    // directly — see WbDrawDispatcherClipSlotTests.
    internal const int ClipSlotCull = -1;

    /// <summary>
    /// Phase U.4: resolve the clip slot for one entity per the slot/gate policy.
    /// Returns <see cref="ClipSlotCull"/> to drop the entity's instances entirely.
    /// <list type="bullet">
    /// <item>ServerGuid != 0 (live dynamic: player / NPC / items / doors) ⇒ slot 0
    /// (UNCLIPPED — retail draws live-dynamic unclipped; depth only).</item>
    /// <item>ParentCellId != null (cell static) ⇒ the cell's slot, or CULL when the
    /// cell isn't in <paramref name="cellIdToSlot"/> (not visible / nothing-visible).</item>
    /// <item>ParentCellId == null (outdoor scenery / building shell) ⇒ the OutsideView
    /// slot when <paramref name="outdoorVisible"/>, else CULL.</item>
    /// </list>
    /// Only called when <c>_clipRoutingActive</c> (indoor root). On the U.3 / outdoor
    /// path every instance is slot 0 and nothing is culled — see
    /// <see cref="ResolveSlotForFrame"/>, which gates on that flag.
    /// <para>
    /// INVARIANT: <paramref name="parentCellId"/> and the keys of
    /// <paramref name="cellIdToSlot"/> MUST live in the same FULL cell-id space
    /// (<c>lbMask | OtherCellId</c>, e.g. <c>0xA9B40164</c>). A bare-low-byte
    /// ParentCellId (e.g. <c>0x64</c>) would never match a full-id key and would
    /// silently CULL every indoor stab — cf. the L.2e bare-low-byte finding in
    /// CLAUDE.md where player CellId was tracked without its landblock prefix.
    /// </para>
    /// <para>
    /// <c>internal static</c> + pure (reads no instance state) so the clip-slot
    /// unit tests exercise every branch without a GL context. The caller hands in
    /// the routing fields it would otherwise read from <c>_cellIdToSlot</c> etc.
    /// </para>
    /// </summary>
    internal static int ResolveEntitySlot(
        uint serverGuid,
        uint? parentCellId,
        IReadOnlyDictionary<uint, int> cellIdToSlot,
        int outdoorSlot,
        bool outdoorVisible)
    {
        // Live-dynamic entities render unclipped regardless of cell — retail draws
        // the player / NPCs / dropped items through the depth buffer without portal
        // clipping. ServerGuid is the live-dynamic marker (0 for dat-hydrated).
        if (serverGuid != 0)
            return 0;

        if (parentCellId is uint parentCell)
            return cellIdToSlot.TryGetValue(parentCell, out int slot) ? slot : ClipSlotCull;

        // Outdoor scenery / building shell (no ParentCellId). Indoor root: gate to
        // the OutsideView slot, or cull when nothing outdoors is visible.
        return outdoorVisible ? outdoorSlot : ClipSlotCull;
    }

    /// <summary>
    /// Phase U.4: the call-site clip-slot decision for one entity, returning the
    /// <c>(Slot, Culled)</c> pair the per-entity loop body consumes. Wraps
    /// <see cref="ResolveEntitySlot"/> with the <paramref name="clipRoutingActive"/>
    /// gate: when routing is INACTIVE (outdoor root / no portal frame), every entity
    /// is slot 0 and nothing is clip-culled — the bit-identical-to-U.3 property, so
    /// the resolver (and <paramref name="cellIdToSlot"/>) is bypassed entirely.
    /// When active, a CULL sentinel maps to <c>(0, culled=true)</c> — the slot value
    /// is never emitted for a culled entity.
    /// <c>internal static</c> + pure so the whole policy (including the routing-
    /// inactive branch) is unit-testable — see WbDrawDispatcherClipSlotTests.
    /// </summary>
    internal static (uint Slot, bool Culled) ResolveSlotForFrame(
        bool clipRoutingActive,
        uint serverGuid,
        uint? parentCellId,
        IReadOnlyDictionary<uint, int>? cellIdToSlot,
        int outdoorSlot,
        bool outdoorVisible)
    {
        if (!clipRoutingActive)
            return (0u, false);

        int resolved = ResolveEntitySlot(serverGuid, parentCellId, cellIdToSlot!, outdoorSlot, outdoorVisible);
        bool culled = resolved == ClipSlotCull;
        return (culled ? 0u : (uint)resolved, culled);
    }

    public static Matrix4x4 ComposePartWorldMatrix(
        Matrix4x4 entityWorld,
        Matrix4x4 animOverride,
        Matrix4x4 restPose)
        => restPose * animOverride * entityWorld;

    /// <summary>
    /// Entry for <see cref="WalkEntities"/> per-landblock iteration.
    /// Mirrors the shape yielded by <c>GpuWorldState.LandblockEntries</c>.
    /// </summary>
    public readonly record struct LandblockEntry(
        uint LandblockId,
        Vector3 AabbMin,
        Vector3 AabbMax,
        IReadOnlyList<WorldEntity> Entities,
        IReadOnlyDictionary<uint, WorldEntity>? AnimatedById);

    /// <summary>
    /// Result of <see cref="WalkEntities"/> — the list of (entity, meshRef index)
    /// pairs that passed all visibility filters, plus a diagnostic walk count.
    /// </summary>
    public struct WalkResult
    {
        public int EntitiesWalked;
        public int BuildingShellAnchorPass;
        public int BuildingShellAnchorReject;
        public List<(WorldEntity Entity, int MeshRefIndex, uint LandblockId)> ToDraw;
    }

    /// <summary>
    /// Pure-CPU visibility filter over <paramref name="landblockEntries"/>.
    /// Separated from <see cref="Draw"/> so tests can exercise it without GL state.
    ///
    /// <para>
    /// A.5 T17 Change #1: when an LB is frustum-culled AND
    /// <paramref name="animatedEntityIds"/> is non-empty, the OLD path walked
    /// every entity in the LB just to find the few animated ones. This helper
    /// fixes that: if the LB is invisible, we iterate
    /// <paramref name="animatedEntityIds"/> directly and look each up in
    /// <c>entry.AnimatedById</c> (typically &lt;50 animated, up to ~10K total).
    /// </para>
    ///
    /// <para>
    /// A.5 T18 Change #2: per-entity AABB cull reads from the cached
    /// <see cref="WorldEntity.AabbMin"/>/<see cref="WorldEntity.AabbMax"/>
    /// (refreshed lazily if <see cref="WorldEntity.AabbDirty"/>), instead of
    /// recomputing Position±5 each frame.
    /// </para>
    /// </summary>
    /// <summary>
    /// Test-friendly overload that allocates a fresh ToDraw list per call.
    /// Production code (<see cref="Draw"/>) uses the no-alloc overload below
    /// with a caller-provided scratch list.
    /// </summary>
    internal static WalkResult WalkEntities(
        IEnumerable<LandblockEntry> landblockEntries,
        FrustumPlanes? frustum,
        uint? neverCullLandblockId,
        HashSet<uint>? visibleCellIds,
        HashSet<uint>? animatedEntityIds)
    {
        var scratch = new List<(WorldEntity Entity, int MeshRefIndex, uint LandblockId)>();
        var result = new WalkResult { ToDraw = scratch };
        WalkEntitiesInto(
            landblockEntries, frustum, neverCullLandblockId,
            visibleCellIds, animatedEntityIds, scratch, ref result);
        return result;
    }

    /// <summary>
    /// No-alloc overload: clears + populates the caller-provided <paramref name="scratch"/>
    /// list. <see cref="Draw"/> reuses a per-dispatcher scratch field across frames to
    /// avoid the 480+ KB / frame GC pressure that the test-friendly overload incurs.
    /// Returns walk count via <paramref name="result"/>'s <c>EntitiesWalked</c> field.
    ///
    /// <para>
    /// When <paramref name="indoorProbeState"/> is non-null the method emits
    /// <c>[indoor-cull]</c> lines for cell entities rejected by the
    /// visibleCellIds or frustum filters, and <c>[indoor-walk]</c> lines for
    /// cell entities that pass all filters. Rate-limited by
    /// <see cref="IndoorProbeState"/>. Pass <see langword="null"/> (the default)
    /// to disable all probe emission — used by the test-friendly
    /// <see cref="WalkEntities"/> overload.
    /// </para>
    /// </summary>
    internal static void WalkEntitiesInto(
        IEnumerable<LandblockEntry> landblockEntries,
        FrustumPlanes? frustum,
        uint? neverCullLandblockId,
        HashSet<uint>? visibleCellIds,
        HashSet<uint>? animatedEntityIds,
        List<(WorldEntity Entity, int MeshRefIndex, uint LandblockId)> scratch,
        ref WalkResult result,
        IndoorProbeState? indoorProbeState = null,
        EntitySet set = EntitySet.All)
    {
        scratch.Clear();
        result.EntitiesWalked = 0;
        result.ToDraw = scratch;

        foreach (var entry in landblockEntries)
        {
            bool landblockVisible = frustum is null
                || entry.LandblockId == neverCullLandblockId
                || FrustumCuller.IsAabbVisible(frustum.Value, entry.AabbMin, entry.AabbMax);

            if (!landblockVisible)
            {
                // A.5 T17 Change #1: walk only animated entities, not all entities.
                // Avoids O(N_entities) scan when only O(N_animated) work is needed.
                if (animatedEntityIds is null || animatedEntityIds.Count == 0) continue;
                if (entry.AnimatedById is null) continue;
                foreach (var animatedId in animatedEntityIds)
                {
                    if (!entry.AnimatedById.TryGetValue(animatedId, out var entity)) continue;
                    // Phase A8: EntitySet partition for indoor/outdoor split passes.
                    if (!EntityMatchesSet(entity, set)) continue;
                    if (entity.MeshRefs.Count == 0) continue;
                    bool shellScoped = IsShellScopedSet(set)
                        && entity.IsBuildingShell
                        && visibleCellIds is not null;
                    if (!EntityPassesVisibleCellGate(entity, visibleCellIds, set))
                    {
                        if (shellScoped) result.BuildingShellAnchorReject++;
                        continue;
                    }
                    if (shellScoped) result.BuildingShellAnchorPass++;
                    result.EntitiesWalked++;
                    for (int i = 0; i < entity.MeshRefs.Count; i++)
                        scratch.Add((entity, i, entry.LandblockId));
                }
                continue;
            }

            foreach (var entity in entry.Entities)
            {
                // Phase A8: EntitySet partition for indoor/outdoor split passes.
                if (!EntityMatchesSet(entity, set)) continue;
                if (entity.MeshRefs.Count == 0) continue;

                // Detect cell entity for indoor probes — first MeshRef.GfxObjId
                // is an EnvCell id (low 16 bits ≥ 0x0100). Cheap to compute;
                // result reused for all probe checks below.
                ulong cellProbeId = (ulong)entity.MeshRefs[0].GfxObjId;
                bool isCellEntity = indoorProbeState is not null
                    && RenderingDiagnostics.IsEnvCellId(cellProbeId);

                bool shellScoped = IsShellScopedSet(set)
                    && entity.IsBuildingShell
                    && visibleCellIds is not null;
                bool cellInVis = EntityPassesVisibleCellGate(entity, visibleCellIds, set);
                if (!cellInVis)
                {
                    if (shellScoped) result.BuildingShellAnchorReject++;
                    if (isCellEntity && RenderingDiagnostics.ProbeIndoorCullEnabled
                        && indoorProbeState!.ShouldEmit(cellProbeId))
                    {
                        Console.WriteLine(
                            $"[indoor-cull] cellEnt=0x{entity.Id:X8} " +
                            $"reason=visibleCellIds-miss " +
                            $"parentCell=0x{entity.ParentCellId!.Value:X8}");
                    }
                    continue;
                }
                if (shellScoped) result.BuildingShellAnchorPass++;

                // Per-entity AABB frustum cull (perf #3). Animated entities bypass —
                // they're tracked at landblock level + need per-frame work regardless.
                // A.5 T18 Change #2: read cached AABB, refresh lazily on AabbDirty.
                bool isAnimated = animatedEntityIds?.Contains(entity.Id) == true;
                bool aabbVisible = true;
                if (frustum is not null && !isAnimated && entry.LandblockId != neverCullLandblockId)
                {
                    if (entity.AabbDirty) entity.RefreshAabb();
                    aabbVisible = FrustumCuller.IsAabbVisible(frustum.Value, entity.AabbMin, entity.AabbMax);
                }

                if (!aabbVisible)
                {
                    if (isCellEntity && RenderingDiagnostics.ProbeIndoorCullEnabled
                        && indoorProbeState!.ShouldEmit(cellProbeId))
                    {
                        Console.WriteLine(
                            $"[indoor-cull] cellEnt=0x{entity.Id:X8} " +
                            $"reason=frustum " +
                            $"aabbMin=({entity.AabbMin.X:F1},{entity.AabbMin.Y:F1},{entity.AabbMin.Z:F1}) " +
                            $"aabbMax=({entity.AabbMax.X:F1},{entity.AabbMax.Y:F1},{entity.AabbMax.Z:F1})");
                    }
                    continue;
                }

                // Passed all filters — emit walk probe.
                if (isCellEntity && RenderingDiagnostics.ProbeIndoorWalkEnabled
                    && indoorProbeState!.ShouldEmit(cellProbeId))
                {
                    Console.WriteLine(
                        $"[indoor-walk] cellEnt=0x{entity.Id:X8} " +
                        $"pos=({entity.Position.X:F1},{entity.Position.Y:F1},{entity.Position.Z:F1}) " +
                        $"parentCell=0x{(entity.ParentCellId ?? 0u):X8} " +
                        $"meshRef0=0x{cellProbeId:X8} " +
                        $"meshRefCount={entity.MeshRefs.Count} " +
                        $"landblockVisible=true aabbVisible=true cellInVis=true");
                }

                result.EntitiesWalked++;
                for (int i = 0; i < entity.MeshRefs.Count; i++)
                    scratch.Add((entity, i, entry.LandblockId));
            }
        }
    }

    public void Draw(
        ICamera camera,
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                     IReadOnlyList<WorldEntity> Entities,
                     IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> landblockEntries,
        FrustumPlanes? frustum = null,
        uint? neverCullLandblockId = null,
        HashSet<uint>? visibleCellIds = null,
        HashSet<uint>? animatedEntityIds = null,
        EntitySet set = EntitySet.All)
    {
        _shader.Use();
        _indoorProbeFrameCounter++;
        var vp = camera.View * camera.Projection;
        _shader.SetMatrix4("uViewProjection", vp);

        bool diag = string.Equals(Environment.GetEnvironmentVariable("ACDREAM_WB_DIAG"), "1", StringComparison.Ordinal);

        if (diag && !_gpuQueriesInitialized)
        {
            for (int i = 0; i < GpuQueryRingDepth; i++)
            {
                _gpuQueryOpaque[i]      = _gl.GenQuery();
                _gpuQueryTransparent[i] = _gl.GenQuery();
            }
            _gpuQueriesInitialized = true;
        }

        // Always run the CPU stopwatch — cheap; only logged under diag.
        _cpuStopwatch.Restart();

        // Camera world-space position for front-to-back sort (perf #2). The view
        // matrix is the inverse of the camera's world transform, so the world
        // translation lives in the inverse's translation row.
        Vector3 camPos = Vector3.Zero;
        if (Matrix4x4.Invert(camera.View, out var invView))
            camPos = invView.Translation;

        // ── Phase 1: clear groups, walk entities, build groups ──────────────
        foreach (var grp in _groups.Values) { grp.Matrices.Clear(); grp.Slots.Clear(); }

        var metaTable = _meshAdapter.MetadataTable;
        uint anyVao = 0;

        // Project the 5-tuple enumerable into LandblockEntry records for WalkEntities.
        static IEnumerable<LandblockEntry> ToEntries(
            IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                         IReadOnlyList<WorldEntity> Entities,
                         IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> src)
        {
            foreach (var e in src)
                yield return new LandblockEntry(e.LandblockId, e.AabbMin, e.AabbMax, e.Entities, e.AnimatedById);
        }

        // A.5 T26 follow-up (Bug B): use the no-alloc WalkEntitiesInto overload
        // that populates _walkScratch (a per-dispatcher field reused across frames)
        // instead of allocating a fresh List<(WorldEntity, int)> per frame.
        //
        // Pass an IndoorProbeState when any indoor probe is active so the static
        // WalkEntitiesInto can emit rate-limited [indoor-cull] / [indoor-walk]
        // lines without needing access to instance fields. Null = probes off.
        IndoorProbeState? probeState = null;
        if (RenderingDiagnostics.ProbeIndoorCullEnabled || RenderingDiagnostics.ProbeIndoorWalkEnabled)
        {
            // _currentFrame is snapped at construction time. Construct
            // once per Draw() call only — a second construction within
            // the same frame would stamp the dictionary with the
            // (already-advanced) counter value, suppressing the second
            // pass's emissions for IndoorProbeRateLimitFrames frames.
            // Today Draw() is called exactly once per frame; if a
            // future refactor adds a shadow / reflection / second pass,
            // this assumption needs revisiting.
            probeState = new IndoorProbeState(_lastIndoorProbeFrame, _indoorProbeFrameCounter);
        }

        var walkResult = default(WalkResult);
        WalkEntitiesInto(
            ToEntries(landblockEntries),
            frustum,
            neverCullLandblockId,
            visibleCellIds,
            animatedEntityIds,
            _walkScratch,
            ref walkResult,
            probeState,
            set);

        // Tier 1 cache (#53) flush-tracking locals. _walkScratch holds one tuple
        // per (entity, MeshRefIndex) and is in entity-order, so all MeshRefs of
        // a given entity are contiguous. We accumulate ALL of an entity's
        // batches into _populateScratch, then flush exactly once per entity:
        // either when the iteration crosses to a different entity, or at the
        // end of the loop for the last entity. Flushing per-tuple would
        // overwrite earlier MeshRefs (the cache is keyed by entity.Id), so
        // multi-part Setup-backed entities would only retain their LAST
        // MeshRef's batches — bug fixed in commit after 2f489a8.
        uint? populateEntityId = null;
        uint populateLandblockId = 0;

        // Tier 1 cache (#53) — fast-path one-shot tracker. The cache stores a
        // FLAT list of batches across all MeshRefs of an entity, so a single
        // ApplyCacheHit call already drew every batch. _walkScratch yields
        // one tuple per (entity, MeshRefIndex), so without this guard a
        // 3-MeshRef static entity on a frame-2 cache hit would call
        // ApplyCacheHit 3 times — appending all 6 batches × 3 = 18 instances
        // to _groups instead of 6. Result: severe Z-fighting + 3× perf hit
        // on every multi-part static entity (buildings, statues, multi-MeshRef
        // NPCs). The fast path must fire only on the FIRST tuple of each
        // entity; subsequent tuples skip via this tracker.
        uint? lastHitEntityId = null;

        // Tier 1 cache (#53) — incomplete-entity guard. When any MeshRef of
        // the current entity has _meshAdapter.TryGetRenderData return null
        // (mesh still async-decoding via ObjectMeshManager.PrepareMeshDataAsync),
        // we mark the entity incomplete and DROP the accumulated populate
        // scratch at entity boundary instead of writing it to the cache.
        // Otherwise the cache would hold a partial classification (some parts
        // missing), and frame-2 cache hits would persist that partial render
        // even after the missing mesh loads — every subsequent frame sees the
        // cache hit and skips re-classification, so the missing parts never
        // recover. User-visible symptom: the drudge statue on top of the
        // Foundry (multi-part Setup entity with AnimPartChange) renders with
        // some parts missing permanently. Reset on entity change.
        bool currentEntityIncomplete = false;

        // Per-tuple entity tracker used purely for entity-change detection.
        // Updated UNCONDITIONALLY at end of every tuple (including tuples that
        // skip via null renderData), so the flag-reset block below correctly
        // distinguishes "new entity" from "same entity, different tuple."
        // populateEntityId can't be used for this because it's only set after
        // a successful slow-path classification.
        uint? prevTupleEntityId = null;

        foreach (var (entity, partIdx, landblockId) in _walkScratch)
        {
            if (diag) _entitiesSeen++;

            // Skip subsequent tuples of an entity that already cache-hit on
            // its first tuple. ApplyCacheHit drew the full flat batch list;
            // re-firing here would N-multiply the instance count. Diag
            // _entitiesDrawn is bumped here to preserve per-tuple parity with
            // the previous counting semantics.
            if (lastHitEntityId == entity.Id)
            {
                if (diag) _entitiesDrawn++;
                continue;
            }

            // Reset the hit tracker on entity change so the next entity's
            // first tuple re-checks the cache. (When this iteration is the
            // FIRST tuple of a new entity after a cache-hit entity, we must
            // not retain the previous entity's id.)
            if (lastHitEntityId.HasValue && lastHitEntityId.Value != entity.Id)
            {
                lastHitEntityId = null;
            }

            // Tier 1 cache (#53) — drop the previous entity's accumulated
            // populate scratch BEFORE MaybeFlushOnEntityChange runs. If the
            // previous entity ended incomplete (≥1 null renderData), we MUST
            // NOT cache its partial classification: clear scratch and null
            // the tracker so MaybeFlushOnEntityChange sees the cleaned state
            // and no-ops for this entity. Reset the incomplete flag for the
            // new entity so each one gets a fresh measurement.
            //
            // CRITICAL: the flag reset must fire ONLY on entity change, not
            // every tuple. Resetting per-tuple within the same entity would
            // undo a null-renderData flag set by a previous tuple of the same
            // entity → if the missing MeshRef sits in the MIDDLE of the
            // entity's MeshRefs list, a later valid tuple's reset would
            // re-mark the entity "complete" and let partial data populate
            // the cache. Trees with [trunk valid, branches null, leaves
            // valid] hit this exactly — branches never recover.
            bool isNewEntity = !prevTupleEntityId.HasValue || prevTupleEntityId.Value != entity.Id;
            if (isNewEntity)
            {
                if (populateEntityId.HasValue && currentEntityIncomplete)
                {
                    _populateScratch.Clear();
                    populateEntityId = null;
                }
                currentEntityIncomplete = false;

                // Phase U.4: resolve this entity's clip slot ONCE per entity
                // (constant across its tuples). On the U.3 / outdoor path
                // (_clipRoutingActive false) every entity is slot 0, never culled.
                // The whole decision (including the routing-active gate) lives in
                // the pure ResolveSlotForFrame helper so it's unit-testable.
                (_currentEntitySlot, _currentEntityCulled) = ResolveSlotForFrame(
                    _clipRoutingActive, entity.ServerGuid, entity.ParentCellId,
                    _cellIdToSlot, _outdoorSlot, _outdoorVisible);
            }
            prevTupleEntityId = entity.Id;

            // Flush-on-entity-change: if the previous entity accumulated any
            // batches AND this iteration is for a different entity, populate
            // its cache entry now and reset the scratch buffer. Runs for ALL
            // entities (including this-entity-culled) so the PREVIOUS entity's
            // cache always flushes at the boundary.
            (populateEntityId, populateLandblockId) = MaybeFlushOnEntityChange(
                populateEntityId, populateLandblockId, entity.Id, _cache, _populateScratch);

            // Phase U.4: a culled entity (cell not visible, or no outdoors visible
            // for an outdoor stab) contributes NO instances. Skip after the
            // boundary flush above so the previous entity still committed; the
            // next entity's isNewEntity logic is unaffected (prevTupleEntityId is
            // already updated). Matches the existing visible-cell / frustum cull:
            // nothing enters _groups, so neither binding=0 nor binding=3 sees it.
            if (_currentEntityCulled)
                continue;

            var entityWorld =
                Matrix4x4.CreateFromQuaternion(entity.Rotation) *
                Matrix4x4.CreateTranslation(entity.Position);

            bool isAnimated = animatedEntityIds?.Contains(entity.Id) == true;

            // Cache-hit fast path (Task 10): static entity with a populated
            // cache entry skips classification entirely. Walk the cached
            // (GroupKey, RestPose) flat list and append cached.RestPose *
            // entityWorld to each matching group's matrices. Animated entities
            // bypass the cache (collector is set null below; their entries are
            // never populated in the first place).
            //
            // Placed AFTER the entity-change flush above so that, on a
            // hit, this iteration also finishes flushing any pending
            // populate state from a previous entity. Animated entities never
            // enter this branch — the !isAnimated guard makes that explicit.
            //
            // Fires ONCE per entity: the first tuple reaches here, runs
            // ApplyCacheHit, sets lastHitEntityId, and continues. Subsequent
            // tuples of the same entity short-circuit at the top of the loop
            // body via the lastHitEntityId == entity.Id check above.
            if (!isAnimated && !_tier1CacheDisabled && _cache.TryGet(entity.Id, landblockId, out var cachedEntry))
            {
                ApplyCacheHit(cachedEntry!, entityWorld, AppendInstanceToGroup);

                // anyVao recovery: when the first visible entity in the frame
                // takes the fast path, no slow-path lookup has populated
                // anyVao yet. Look up THIS entity's first MeshRef once via
                // the mesh adapter — cheap dict lookup, not a re-classify.
                if (anyVao == 0)
                {
                    var firstMeshRef = entity.MeshRefs[partIdx];
                    var firstRenderData = _meshAdapter.TryGetRenderData(firstMeshRef.GfxObjId);
                    if (firstRenderData is not null) anyVao = firstRenderData.VAO;
                }

                if (diag) _entitiesDrawn++;
                lastHitEntityId = entity.Id;

#if DEBUG
                // Cross-check guard: assert the membership predicate held at hit time.
                // The full re-classification cross-check (spec section 6.5) is a stretch
                // goal; this simpler assert catches the prior Tier 1 bug class — a
                // static entity that turns out to actually be animated would fire here.
                //
                // Structurally redundant with the `if (!isAnimated && ...)` branch
                // condition, but serves as a TRIPWIRE: a future refactor that
                // incorrectly relaxes the branch condition (e.g., removes
                // `!isAnimated` from the guard) would silently allow animated
                // entities into the fast path; the assert catches that immediately.
                System.Diagnostics.Debug.Assert(
                    !isAnimated,
                    $"EntityClassificationCache hit on animated entity {entity.Id} — invariant violated");
#endif

                continue;
            }

            // Compute palette-override hash ONCE per entity (perf #4).
            // Reused across every (part, batch) lookup so the FNV-1a fold
            // over SubPalettes runs once instead of N times. Zero when the
            // entity has no palette override (trees, scenery).
            ulong palHash = 0;
            if (entity.PaletteOverride is not null)
                palHash = TextureCache.HashPaletteOverride(entity.PaletteOverride);

            // Note: GameWindow's spawn path already applies
            // AnimPartChanges + GfxObjDegradeResolver (Issue #47 fix —
            // close-detail mesh swap for humanoids) to MeshRefs. We
            // trust MeshRefs as the source of truth here. AnimatedEntityState's
            // overrides become relevant only for hot-swap (0xF625
            // ObjDescEvent) which today rebuilds MeshRefs anyway.
            var meshRef = entity.MeshRefs[partIdx];
            ulong gfxObjId = meshRef.GfxObjId;

            var renderData = _meshAdapter.TryGetRenderData(gfxObjId);

            // [indoor-lookup] probe — emit once per cell entity per sec.
            // Fires BEFORE the null-renderData early-continue so a miss still
            // emits hit=false, distinguishing H2 (empty batches) from H6
            // (dispatcher fails to traverse Setup).
            ulong lookupCellId = (ulong)gfxObjId;
            if (RenderingDiagnostics.IsEnvCellId(lookupCellId)
                && RenderingDiagnostics.ProbeIndoorLookupEnabled
                // Rate-limit in a separate namespace from [indoor-walk]/[indoor-cull]
                // (which key on the same gfxObjId). Without this, IndoorAll=1 would
                // silence the lookup probe whenever the walk probe fired first.
                && ShouldEmitIndoorProbe(lookupCellId | 0x8000_0000_0000_0000UL))
            {
                bool hit = renderData is not null;
                bool isSetup = hit && renderData!.IsSetup;
                int partCount = isSetup ? renderData!.SetupParts.Count : 0;

                int partsHit = 0, partsMiss = 0;
                if (isSetup)
                {
                    foreach (var (partId, _) in renderData!.SetupParts)
                    {
                        if (_meshAdapter.TryGetRenderData(partId) is not null) partsHit++;
                        else partsMiss++;
                    }
                }

                bool hasEnvCellGeom = isSetup
                    && renderData!.SetupParts.Exists(t => (t.GfxObjId & 0x1_0000_0000UL) != 0);

                Console.WriteLine(
                    $"[indoor-lookup] cellId=0x{lookupCellId:X8} " +
                    $"hit={hit} isSetup={isSetup} partCount={partCount} " +
                    $"hasEnvCellGeom={hasEnvCellGeom} partsHit={partsHit} partsMiss={partsMiss}");
            }

            if (renderData is null)
            {
                // Tier 1 cache (#53): mesh data is still async-decoding via
                // WB's ObjectMeshManager.PrepareMeshDataAsync. Flag the entity
                // as incomplete so the entity-boundary check (or end-of-loop
                // check) drops the accumulated populate scratch instead of
                // caching a partial classification. The slow path retries on
                // the next frame; once all this entity's meshes have loaded,
                // the populate fires with the complete batch set.
                currentEntityIncomplete = true;
                if (diag) _meshesMissing++;
                continue;
            }
            if (anyVao == 0) anyVao = renderData.VAO;

            // Cache-miss path (animated entities skip cache entirely).
            // Static entities accumulate into _populateScratch across ALL
            // their MeshRefs; the flush at next-entity-boundary (or
            // end-of-loop) commits them as a single Populate call.
            var collector = isAnimated ? null : _populateScratch;

            bool drewAny = false;
            if (renderData.IsSetup && renderData.SetupParts.Count > 0)
            {
                foreach (var (partGfxObjId, partTransform) in renderData.SetupParts)
                {
                    var partData = _meshAdapter.TryGetRenderData(partGfxObjId);
                    if (partData is null) continue;

                    var model = ComposePartWorldMatrix(
                        entityWorld, meshRef.PartTransform, partTransform);

                    // [indoor-xform] probe — only for the cell's synthetic
                    // geometry part (bit 32 set, per WB's PrepareEnvCellMeshData
                    // cellGeomId convention). One line per part per sec.
                    // Disambiguates hypothesis H5 (transform double-apply —
                    // composedT lands at 2 × cellOrigin).
                    if ((partGfxObjId & 0x1_0000_0000UL) != 0
                        && RenderingDiagnostics.ProbeIndoorXformEnabled
                        && ShouldEmitIndoorProbe(partGfxObjId))
                    {
                        Console.WriteLine(
                            $"[indoor-xform] cellGeomId=0x{partGfxObjId:X16} " +
                            $"entityWorldT=({entityWorld.Translation.X:F2},{entityWorld.Translation.Y:F2},{entityWorld.Translation.Z:F2}) " +
                            $"meshRefT=({meshRef.PartTransform.Translation.X:F2},{meshRef.PartTransform.Translation.Y:F2},{meshRef.PartTransform.Translation.Z:F2}) " +
                            $"partT=({partTransform.Translation.X:F2},{partTransform.Translation.Y:F2},{partTransform.Translation.Z:F2}) " +
                            $"composedT=({model.Translation.X:F2},{model.Translation.Y:F2},{model.Translation.Z:F2})");
                    }

                    var restPose = partTransform * meshRef.PartTransform;
                    ClassifyBatches(partData, partGfxObjId, model, entity, meshRef, palHash, metaTable, restPose, collector);
                    drewAny = true;
                }
            }
            else
            {
                var model = meshRef.PartTransform * entityWorld;
                ClassifyBatches(renderData, gfxObjId, model, entity, meshRef, palHash, metaTable, restPose: meshRef.PartTransform, collector: collector);
                drewAny = true;
            }

            // Track THIS entity for the next iteration's flush check. Only
            // when collector is non-null (entity is static); animated entities
            // leave the tracker null so we don't try to flush them.
            if (collector is not null)
            {
                populateEntityId = entity.Id;
                populateLandblockId = landblockId;
            }

            if (diag && drewAny) _entitiesDrawn++;
        }

        // Tier 1 cache (#53) — drop the accumulated populate scratch if the
        // LAST entity in the loop ended incomplete (had ≥1 null renderData).
        // Same reason as the entity-boundary handling above: avoid caching a
        // partial classification. The slow path will retry on the next frame
        // and populate correctly once all meshes have loaded.
        if (currentEntityIncomplete)
        {
            _populateScratch.Clear();
            populateEntityId = null;
        }

        // Final flush: the last entity in _walkScratch has no "next iteration"
        // to trigger the entity-change flush, so commit its accumulated batches
        // here. No-op when the last entity was animated (populateEntityId stays
        // null) or when no entities walked at all.
        FinalFlushPopulate(populateEntityId, populateLandblockId, _cache, _populateScratch);

        // Nothing visible — skip the GL pass entirely.
        if (anyVao == 0)
        {
            LastDrawStats = new DrawStats(set, walkResult.EntitiesWalked, _walkScratch.Count, 0, 0, 0, 0, 0, 0);
            _cpuStopwatch.Stop();
            if (diag) MaybeFlushDiag();
            return;
        }

        // ── Phase 3: assign FirstInstance per group, lay matrices contiguously, sort opaque ──
        int totalInstances = 0;
        foreach (var grp in _groups.Values) totalInstances += grp.Matrices.Count;
        if (totalInstances == 0)
        {
            LastDrawStats = new DrawStats(set, walkResult.EntitiesWalked, _walkScratch.Count, 0, 0, 0, 0, 0, 0);
            _cpuStopwatch.Stop();
            if (diag) MaybeFlushDiag();
            return;
        }

        int needed = totalInstances * 16;
        if (_instanceData.Length < needed)
            _instanceData = new float[needed + 256 * 16];

        // Phase U.4: size the per-instance clip-slot buffer to match the instance
        // count and lay it out in the SAME group order / cursor as _instanceData,
        // so instanceClipSlot[i] (binding=3) tracks Instances[i] (binding=0). On
        // the U.3 / outdoor path every Slots entry is 0 ⇒ identical to U.3.
        if (_clipSlotData.Length < totalInstances)
            _clipSlotData = new uint[totalInstances + 256];

        _opaqueDraws.Clear();
        _translucentDraws.Clear();

        int cursor = 0;
        foreach (var grp in _groups.Values)
        {
            if (grp.Matrices.Count == 0) continue;

            grp.FirstInstance = cursor;
            grp.InstanceCount = grp.Matrices.Count;

            // Use the first instance's translation as the group's representative
            // position for front-to-back sort (perf #2). Cheap heuristic; works
            // well when instances of one group are spatially coherent
            // (typical for trees in one landblock area, NPCs at one spawn).
            var first = grp.Matrices[0];
            var grpPos = new Vector3(first.M41, first.M42, first.M43);
            grp.SortDistance = Vector3.DistanceSquared(camPos, grpPos);

            for (int i = 0; i < grp.Matrices.Count; i++)
            {
                WriteMatrix(_instanceData, cursor * 16, grp.Matrices[i]);
                // Slots[] is parallel to Matrices[] within the group; write the
                // slot at the same cursor so binding=3 stays aligned with binding=0.
                _clipSlotData[cursor] = grp.Slots[i];
                cursor++;
            }

            if (IsOpaque(grp.Translucency))
                _opaqueDraws.Add(grp);
            else
                _translucentDraws.Add(grp);
        }

        // Front-to-back sort within each cull mode. DrawIndirectRange must
        // split MDI calls whenever CullMode changes because GL state is not
        // part of an indirect command. Sorting by distance alone can turn a
        // stable 1k-draw live scene into hundreds of tiny MDI runs after a
        // landblock transition, which shows up as a GPU-command bottleneck
        // without a triangle-count spike.
        _opaqueDraws.Sort(CompareOpaqueSubmissionOrder);
        _translucentDraws.Sort(CompareTransparentSubmissionOrder);

        // ── Phase 4: build IndirectGroupInput list (opaque sorted, then translucent),
        //            fill via BuildIndirectArrays ──────────────────────────────────
        int totalDraws = _opaqueDraws.Count + _translucentDraws.Count;
        if (_batchData.Length < totalDraws)
            _batchData = new BatchData[totalDraws + 64];
        if (_indirectCommands.Length < totalDraws)
            _indirectCommands = new DrawElementsIndirectCommand[totalDraws + 64];
        if (_drawCullModes.Length < totalDraws)
            _drawCullModes = new CullMode[totalDraws + 64];

        var groupInputs = new List<IndirectGroupInput>(totalDraws);
        foreach (var g in _opaqueDraws) groupInputs.Add(ToInput(g));
        foreach (var g in _translucentDraws) groupInputs.Add(ToInput(g));

        // Cast _batchData (private BatchData) to public-mirror BatchDataPublic for BuildIndirectArrays.
        // Layout is asserted at test time (BatchDataPublic_LayoutMatchesPrivateBatchData test).
        var batchPublic = new BatchDataPublic[totalDraws];
        var layout = BuildIndirectArrays(groupInputs, _indirectCommands, batchPublic, _drawCullModes);
        long totalTriangles = 0;
        foreach (var input in groupInputs)
            totalTriangles += (long)(input.IndexCount / 3) * input.InstanceCount;
        int cullRuns =
            CountCullRuns(_drawCullModes, 0, layout.OpaqueCount) +
            CountCullRuns(_drawCullModes, layout.OpaqueCount, layout.TransparentCount);

        // Copy back into _batchData
        for (int i = 0; i < totalDraws; i++)
        {
            _batchData[i] = new BatchData
            {
                TextureHandle = batchPublic[i].TextureHandle,
                TextureLayer  = batchPublic[i].TextureLayer,
                Flags         = batchPublic[i].Flags,
            };
        }
        _opaqueDrawCount       = layout.OpaqueCount;
        _transparentDrawCount  = layout.TransparentCount;
        _transparentByteOffset = layout.TransparentByteOffset;
        LastDrawStats = new DrawStats(
            set,
            walkResult.EntitiesWalked,
            _walkScratch.Count,
            totalInstances,
            totalDraws,
            cullRuns,
            _opaqueDrawCount,
            _transparentDrawCount,
            totalTriangles);

        // ── Phase 5: upload four buffers ────────────────────────────────────
        fixed (float* ip = _instanceData)
            UploadSsbo(_instanceSsbo, 0, ip, totalInstances * 16 * sizeof(float));

        fixed (BatchData* bp = _batchData)
            UploadSsbo(_batchSsbo, 1, bp, totalDraws * sizeof(BatchData));

        // Phase U.4: per-instance clip-slot buffer (binding=3), one uint per
        // instance, laid out parallel to _instanceData in Phase 3's group loop so
        // instanceClipSlot[instanceIndex] tracks Instances[instanceIndex]. On the
        // U.3 / outdoor path every entry is 0 ⇒ slot 0 ⇒ no-clip (identical to
        // U.3); under indoor routing it holds the per-instance slot from
        // ResolveEntitySlot. No clear here — Phase 3 wrote exactly totalInstances
        // entries; only [0..totalInstances) is uploaded, so any stale tail is
        // never read by the shader (BaseInstance + gl_InstanceID < totalInstances).
        fixed (uint* sp = _clipSlotData)
            UploadSsbo(_clipSlotSsbo, 3, sp, totalInstances * sizeof(uint));

        fixed (DrawElementsIndirectCommand* cp = _indirectCommands)
        {
            _gl.BindBuffer(BufferTargetARB.DrawIndirectBuffer, _indirectBuffer);
            _gl.BufferData(BufferTargetARB.DrawIndirectBuffer,
                (nuint)(totalDraws * sizeof(DrawElementsIndirectCommand)), cp, BufferUsageARB.DynamicDraw);
        }

        // Phase U.3: bind the SHARED per-cell clip-region SSBO (binding=2). The
        // GameWindow-level ClipFrame already uploaded + bound it this frame; we
        // re-bind defensively in case another consumer touched binding=2 since.
        // When no shared id is set (0), bind our own no-clip fallback so the
        // shader never reads an unbound SSBO at binding=2.
        BindClipRegionBinding2();

        // ── Phase 6: bind global VAO once ───────────────────────────────────
        _gl.BindVertexArray(anyVao);

        if (string.Equals(Environment.GetEnvironmentVariable("ACDREAM_NO_CULL"), "1", StringComparison.Ordinal))
            _gl.Disable(EnableCap.CullFace);

        // GPU timing: compute this frame's ring slot. We read frame N-3's
        // result (the oldest data in the ring) before overwriting it with
        // frame N's queries. Hoisted to function scope so both the opaque
        // and transparent passes below can reference gpuQuerySlot. See spec
        // §3 Q1/Q2 + §4 in
        // docs/superpowers/specs/2026-05-11-phase-n6-slice1-design.md.
        int gpuQuerySlot = _gpuQueryFrameIndex % GpuQueryRingDepth;
        // diag is part of the gate so the read/issue/increment trio stays
        // symmetric — without it, toggling ACDREAM_WB_DIAG mid-session would
        // freeze the frame counter (gated by diag below) while the read kept
        // re-reading the same slot, producing duplicate stale samples.
        if (diag && _gpuQueriesInitialized && _gpuQueryFrameIndex >= GpuQueryRingDepth)
        {
            _gl.GetQueryObject(_gpuQueryOpaque[gpuQuerySlot], QueryObjectParameterName.ResultAvailable, out int avail);
            if (avail != 0)
            {
                _gl.GetQueryObject(_gpuQueryOpaque[gpuQuerySlot],      QueryObjectParameterName.Result, out ulong opaqueNs);
                _gl.GetQueryObject(_gpuQueryTransparent[gpuQuerySlot], QueryObjectParameterName.Result, out ulong transNs);
                long gpuUs = (long)((opaqueNs + transNs) / 1000UL);
                _gpuSamples[_gpuSampleCursor] = gpuUs;
                _gpuSampleCursor = (_gpuSampleCursor + 1) % _gpuSamples.Length;
            }
            // If avail==0 the sample is dropped silently. MedianMicros
            // computes over the non-zero subset, so dropped samples don't
            // poison the median.
        }

        // ── Phase 7: opaque pass ─────────────────────────────────────────────
        if (_opaqueDrawCount > 0)
        {
            _gl.Disable(EnableCap.Blend);
            _gl.DepthMask(true);
            // A.5 T20: enable A2C for ClipMap foliage — GPU derives sample mask
            // from the alpha written by mesh_modern.frag so foliage edges are
            // smooth under MSAA 4x. A no-op for fully-opaque (α=1) batches.
            // A.5 T22.5: gated by AlphaToCoverage property so Low/Medium presets
            // (no MSAA) skip the unnecessary GL state change.
            if (AlphaToCoverage) _gl.Enable(EnableCap.SampleAlphaToCoverage);
            _shader.SetInt("uRenderPass", 0);
            // Phase Post-A.5 (ISSUE #52, 2026-05-10): opaque section of
            // Batches[] starts at index 0. See uDrawIDOffset comment in
            // mesh_modern.vert for why this is needed.
            _shader.SetInt("uDrawIDOffset", 0);
            _gl.BindBuffer(BufferTargetARB.DrawIndirectBuffer, _indirectBuffer);
            if (diag && _gpuQueriesInitialized) _gl.BeginQuery(QueryTarget.TimeElapsed, _gpuQueryOpaque[gpuQuerySlot]);
            DrawIndirectRange(0, _opaqueDrawCount);
            if (diag && _gpuQueriesInitialized) _gl.EndQuery(QueryTarget.TimeElapsed);
            if (AlphaToCoverage) _gl.Disable(EnableCap.SampleAlphaToCoverage);
        }

        // ── Phase 8: transparent pass ────────────────────────────────────────
        if (_transparentDrawCount > 0)
        {
            _gl.Enable(EnableCap.Blend);
            _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
            _gl.DepthMask(false);
            // Phase Post-A.5 (ISSUE #52, 2026-05-10): transparent section of
            // Batches[] starts at index _opaqueDrawCount. Without this offset,
            // each transparent draw reads BatchData[0..transparentCount) — the
            // OPAQUE section — and the lifestone crystal's apparent texture
            // flickers to whatever opaque batch sorted first that frame. See
            // uDrawIDOffset comment in mesh_modern.vert.
            _shader.SetInt("uDrawIDOffset", _opaqueDrawCount);
            // Closed-shell translucent meshes still need culling, but the
            // cull side must come from each dat batch just like the opaque
            // section. BuildIndirectArrays preserves CullMode in _drawCullModes.
            _gl.FrontFace(FrontFaceDirection.CW);
            _shader.SetInt("uRenderPass", 1);
            if (diag && _gpuQueriesInitialized) _gl.BeginQuery(QueryTarget.TimeElapsed, _gpuQueryTransparent[gpuQuerySlot]);
            DrawIndirectRange(_opaqueDrawCount, _transparentDrawCount);
            if (diag && _gpuQueriesInitialized) _gl.EndQuery(QueryTarget.TimeElapsed);
            _gl.DepthMask(true);
            _gl.Disable(EnableCap.Blend);
        }

        _gl.Disable(EnableCap.CullFace);
        _gl.BindVertexArray(0);

        _cpuStopwatch.Stop();

        if (diag)
        {
            long cpuUs = _cpuStopwatch.ElapsedTicks * 1_000_000L / System.Diagnostics.Stopwatch.Frequency;
            _cpuSamples[_cpuSampleCursor] = cpuUs;
            _cpuSampleCursor = (_cpuSampleCursor + 1) % _cpuSamples.Length;

            // GPU sample read happens BEFORE issuing the next frame's queries
            // (see step 1.3 above). Increment the frame counter here so the
            // next call computes a fresh slot.
            if (_gpuQueriesInitialized) _gpuQueryFrameIndex++;

            _drawsIssued     += _opaqueDrawCount + _transparentDrawCount;
            _instancesIssued += totalInstances;
            MaybeFlushDiag();
        }
    }

    /// <summary>
    /// Phase A8 RR5 (2026-05-26): per-building draw overload. Walks only
    /// entities whose ParentCellId is in <paramref name="cellIds"/>, plus
    /// outdoor-style entities matching the EntitySet partition. Used by
    /// the indoor render branch to scope rendering to the camera-buildings'
    /// cells.
    ///
    /// <para>Mirrors the existing visibleCellIds-based Draw but with an
    /// explicit cell list (not the BFS-derived visibility set). The semantic
    /// difference is at the caller: cellIds = the camera-buildings' EnvCellIds,
    /// not the portal BFS result. The dispatcher's internal logic is identical
    /// — it filters indoor entities by membership in the provided set.</para>
    /// </summary>
    public void Draw(
        ICamera camera,
        IEnumerable<(uint LandblockId, Vector3 AabbMin, Vector3 AabbMax,
                     IReadOnlyList<WorldEntity> Entities,
                     IReadOnlyDictionary<uint, WorldEntity>? AnimatedById)> landblockEntries,
        IReadOnlyCollection<uint> cellIds,
        FrustumPlanes? frustum = null,
        uint? neverCullLandblockId = null,
        HashSet<uint>? animatedEntityIds = null,
        EntitySet set = EntitySet.All)
    {
        // Adapt IReadOnlyCollection<uint> → HashSet<uint> for the existing path.
        // If the caller already passed a HashSet, avoid re-wrapping.
        HashSet<uint> cellIdSet = cellIds is HashSet<uint> hs ? hs : new HashSet<uint>(cellIds);
        Draw(camera, landblockEntries,
             frustum: frustum,
             neverCullLandblockId: neverCullLandblockId,
             visibleCellIds: cellIdSet,
             animatedEntityIds: animatedEntityIds,
             set: set);
    }

    private static IndirectGroupInput ToInput(InstanceGroup g) => new(
        IndexCount:    g.IndexCount,
        FirstIndex:    g.FirstIndex,
        BaseVertex:    g.BaseVertex,
        InstanceCount: g.InstanceCount,
        FirstInstance: g.FirstInstance,
        TextureHandle: g.BindlessTextureHandle,
        TextureLayer:  g.TextureLayer,
        Translucency:  g.Translucency,
        CullMode:      g.CullMode);

    private static int CompareOpaqueSubmissionOrder(InstanceGroup a, InstanceGroup b)
    {
        int cull = a.CullMode.CompareTo(b.CullMode);
        return cull != 0 ? cull : a.SortDistance.CompareTo(b.SortDistance);
    }

    private static int CompareTransparentSubmissionOrder(InstanceGroup a, InstanceGroup b)
    {
        int cull = a.CullMode.CompareTo(b.CullMode);
        return cull != 0 ? cull : b.SortDistance.CompareTo(a.SortDistance);
    }

    private static int CountCullRuns(CullMode[] modes, int startCommand, int commandCount)
    {
        if (commandCount <= 0) return 0;

        int end = startCommand + commandCount;
        int runs = 1;
        var previous = modes[startCommand];
        for (int i = startCommand + 1; i < end; i++)
        {
            var current = modes[i];
            if (current == previous) continue;
            runs++;
            previous = current;
        }
        return runs;
    }

    private unsafe void DrawIndirectRange(int startCommand, int commandCount)
    {
        int end = startCommand + commandCount;
        int command = startCommand;
        while (command < end)
        {
            var cullMode = _drawCullModes[command];
            ApplyCullMode(cullMode);

            int runCount = 1;
            while (command + runCount < end && _drawCullModes[command + runCount] == cullMode)
                runCount++;

            // Each glMultiDrawElementsIndirect call restarts gl_DrawID at 0.
            // Because this method splits one logical opaque/transparent pass
            // into CullMode runs, the shader must receive the absolute command
            // index for this run or it will read BatchData[0] again and bind
            // the wrong texture for later runs.
            _shader.SetInt("uDrawIDOffset", command);
            _gl.MultiDrawElementsIndirect(
                PrimitiveType.Triangles,
                DrawElementsType.UnsignedShort,
                (void*)(command * DrawCommandStride),
                (uint)runCount,
                (uint)DrawCommandStride);

            command += runCount;
        }
    }

    private void ApplyCullMode(CullMode mode)
    {
        // WB BaseObjectRenderManager.cs:850-866 applies CullMode per MDI group.
        // WB GameScene.cs:843 sets FrontFace(CW) globally; SetCullMode then
        // only chooses front/back culling. Keep the same convention here so
        // splitting MDI commands by CullMode cannot resurrect stale CCW state.
        _gl.FrontFace(FrontFaceDirection.CW);
        switch (mode)
        {
            case CullMode.None:
                _gl.Disable(EnableCap.CullFace);
                break;
            case CullMode.Clockwise:
                _gl.Enable(EnableCap.CullFace);
                _gl.CullFace(TriangleFace.Front);
                break;
            case CullMode.CounterClockwise:
            case CullMode.Landblock:
                _gl.Enable(EnableCap.CullFace);
                _gl.CullFace(TriangleFace.Back);
                break;
        }
    }

    private unsafe void UploadSsbo(uint ssbo, uint binding, void* data, int byteCount)
    {
        _gl.BindBuffer(BufferTargetARB.ShaderStorageBuffer, ssbo);
        _gl.BufferData(BufferTargetARB.ShaderStorageBuffer, (nuint)byteCount, data, BufferUsageARB.DynamicDraw);
        _gl.BindBufferBase(BufferTargetARB.ShaderStorageBuffer, binding, ssbo);
    }

    /// <summary>
    /// Phase U.3: bind the per-cell clip-region SSBO to binding=2. Prefers the
    /// shared <see cref="ClipFrame"/> buffer (set via <see cref="SetClipRegionSsbo"/>);
    /// otherwise lazily creates + binds a one-slot no-clip fallback so the shader
    /// never reads an unbound SSBO. The fallback's single slot has count 0
    /// (pass-all), matching <see cref="ClipFrame.NoClip"/>'s slot 0.
    /// </summary>
    private unsafe void BindClipRegionBinding2()
    {
        if (_sharedClipRegionSsbo != 0)
        {
            _gl.BindBufferBase(BufferTargetARB.ShaderStorageBuffer,
                ClipFrame.MeshClipSsboBinding, _sharedClipRegionSsbo);
            return;
        }

        if (_fallbackClipRegionSsbo == 0)
        {
            _fallbackClipRegionSsbo = _gl.GenBuffer();
            // One CellClip slot, all zeros: count 0 ⇒ shader passes every plane.
            var zero = stackalloc byte[ClipFrame.CellClipStrideBytes];
            for (int i = 0; i < ClipFrame.CellClipStrideBytes; i++) zero[i] = 0;
            _gl.BindBuffer(BufferTargetARB.ShaderStorageBuffer, _fallbackClipRegionSsbo);
            _gl.BufferData(BufferTargetARB.ShaderStorageBuffer,
                (nuint)ClipFrame.CellClipStrideBytes, zero, BufferUsageARB.DynamicDraw);
        }
        _gl.BindBufferBase(BufferTargetARB.ShaderStorageBuffer,
            ClipFrame.MeshClipSsboBinding, _fallbackClipRegionSsbo);
    }

    private void MaybeFlushDiag()
    {
        long now = Environment.TickCount64;
        if (now - _lastLogTick > 5000)
        {
            long cpuMed = MedianMicros(_cpuSamples);
            long cpuP95 = Percentile95Micros(_cpuSamples);
            long gpuMed = MedianMicros(_gpuSamples);
            long gpuP95 = Percentile95Micros(_gpuSamples);
            // A.5 T23: flag when entity dispatcher median exceeds 2.0ms budget
            // (Phase A.5 spec §2 acceptance criterion 6). Grep-friendly prefix.
            const long BudgetUs = 2000;
            string budgetFlag = cpuMed > BudgetUs ? " BUDGET_OVER" : "";
            Console.WriteLine(
                $"[WB-DIAG]{budgetFlag} entSeen={_entitiesSeen} entDrawn={_entitiesDrawn} meshMissing={_meshesMissing} drawsIssued={_drawsIssued} instances={_instancesIssued} groups={_groups.Count} " +
                $"cpu_us={cpuMed}m/{cpuP95}p95 gpu_us={gpuMed}m/{gpuP95}p95");
            _entitiesSeen = _entitiesDrawn = _meshesMissing = _drawsIssued = _instancesIssued = 0;
            _lastLogTick = now;
            // Don't reset the sample buffers — they're a moving window of the
            // last 256 frames; clearing per 5s flush would lose recent history.
        }
    }

    private static long MedianMicros(long[] samples)
    {
        var copy = (long[])samples.Clone();
        Array.Sort(copy);
        int nz = 0;
        foreach (var v in copy) if (v > 0) nz++;
        if (nz == 0) return 0;
        return copy[copy.Length - nz / 2];
    }

    private static long Percentile95Micros(long[] samples)
    {
        var copy = (long[])samples.Clone();
        Array.Sort(copy);
        int nz = 0;
        foreach (var v in copy) if (v > 0) nz++;
        if (nz == 0) return 0;
        int idx = copy.Length - 1 - (int)(nz * 0.05);
        return copy[idx];
    }

    // ── Tier 1 cache (#53) helpers extracted for testability ─────────────────
    //
    // Three pure-CPU static helpers carved out of Draw's per-entity loop so
    // unit tests can exercise the populate/flush algorithm + cache-hit fast
    // path without needing a real GL context. Production code (Draw) calls
    // these helpers; the dispatcher integration tests in
    // WbDrawDispatcherBucketingTests use them to drive the same algorithm
    // through deterministic inputs.

    /// <summary>
    /// Apply a cache hit's batches into the per-frame group dictionary by
    /// composing <c>cached.RestPose * entityWorld</c> per batch and routing
    /// the result through <paramref name="appendInstance"/>. The delegate
    /// abstracts over <see cref="InstanceGroup"/> so this helper stays
    /// GL-free and unit-testable.
    /// </summary>
    /// <remarks>
    /// Matrix multiplication is non-commutative: it MUST be
    /// <c>RestPose * entityWorld</c>, not the reverse. See
    /// <see cref="ComposePartWorldMatrix"/> for the full part-world product.
    /// </remarks>
    internal static void ApplyCacheHit(
        EntityCacheEntry entry,
        Matrix4x4 entityWorld,
        Action<GroupKey, Matrix4x4> appendInstance)
    {
        foreach (var cached in entry.Batches)
        {
            appendInstance(cached.Key, cached.RestPose * entityWorld);
        }
    }

    /// <summary>
    /// Per-tuple flush check. If <paramref name="populateEntityId"/> is set
    /// AND differs from <paramref name="currentEntityId"/>, the previous
    /// entity's accumulated batches are committed to <paramref name="cache"/>
    /// and <paramref name="populateScratch"/> is cleared. Returns the
    /// updated tracker tuple — pass these back into the field locals in the
    /// caller's loop.
    /// </summary>
    /// <remarks>
    /// This is the bug-fix structure from commit 00fa8ae (per-MeshRef
    /// Populate would overwrite earlier MeshRefs because the cache is
    /// keyed by entity.Id; flushing only on entity boundary preserves all
    /// MeshRefs' batches). _walkScratch is in entity-order so all MeshRefs
    /// of one entity arrive contiguously.
    /// </remarks>
    internal static (uint? PopulateEntityId, uint PopulateLandblockId)
        MaybeFlushOnEntityChange(
            uint? populateEntityId,
            uint populateLandblockId,
            uint currentEntityId,
            EntityClassificationCache cache,
            List<CachedBatch> populateScratch)
    {
        if (populateEntityId.HasValue && populateEntityId.Value != currentEntityId)
        {
            if (populateScratch.Count > 0)
            {
                cache.Populate(populateEntityId.Value, populateLandblockId, populateScratch.ToArray());
            }
            populateScratch.Clear();
            return (null, 0u);
        }
        return (populateEntityId, populateLandblockId);
    }

    /// <summary>
    /// End-of-loop final flush. The last entity in <c>_walkScratch</c> has
    /// no next-iteration to trigger <see cref="MaybeFlushOnEntityChange"/>,
    /// so commit its accumulated batches here. No-op when no populate is
    /// pending (the last entity was animated, or the scratch is empty).
    /// <para>
    /// End-of-loop only — does NOT reset the caller's tracker locals
    /// (intentional, since they go out of scope immediately after).
    /// </para>
    /// </summary>
    internal static void FinalFlushPopulate(
        uint? populateEntityId,
        uint populateLandblockId,
        EntityClassificationCache cache,
        List<CachedBatch> populateScratch)
    {
        if (populateEntityId.HasValue && populateScratch.Count > 0)
        {
            cache.Populate(populateEntityId.Value, populateLandblockId, populateScratch.ToArray());
            populateScratch.Clear();
        }
    }

    /// <summary>
    /// Instance-side helper used by <see cref="ApplyCacheHit"/>. Looks up or
    /// creates an <see cref="InstanceGroup"/> for the given key in
    /// <c>_groups</c> and appends the per-instance world matrix.
    /// </summary>
    private void AppendInstanceToGroup(GroupKey key, Matrix4x4 model)
    {
        if (!_groups.TryGetValue(key, out var grp))
        {
            grp = new InstanceGroup
            {
                Ibo = key.Ibo,
                FirstIndex = key.FirstIndex,
                BaseVertex = key.BaseVertex,
                IndexCount = key.IndexCount,
                BindlessTextureHandle = key.BindlessTextureHandle,
                TextureLayer = key.TextureLayer,
                Translucency = key.Translucency,
                CullMode = key.CullMode,
            };
            _groups[key] = grp;
        }
        grp.Matrices.Add(model);
        grp.Slots.Add(_currentEntitySlot); // Phase U.4 — parallel to Matrices
    }

    private void ClassifyBatches(
        ObjectRenderData renderData,
        ulong gfxObjId,
        Matrix4x4 model,
        WorldEntity entity,
        MeshRef meshRef,
        ulong palHash,
        AcSurfaceMetadataTable metaTable,
        Matrix4x4 restPose,
        List<CachedBatch>? collector = null)
    {
        for (int batchIdx = 0; batchIdx < renderData.Batches.Count; batchIdx++)
        {
            var batch = renderData.Batches[batchIdx];

            TranslucencyKind translucency;
            if (metaTable.TryLookup(gfxObjId, batchIdx, out var meta))
            {
                translucency = meta.Translucency;
            }
            else
            {
                translucency = batch.IsAdditive ? TranslucencyKind.Additive
                    : batch.IsTransparent ? TranslucencyKind.AlphaBlend
                    : TranslucencyKind.Opaque;
            }

            ulong texHandle = ResolveTexture(entity, meshRef, batch, palHash);
            if (texHandle == 0) continue;

            // TextureLayer is always 0 for per-instance composites; non-zero when
            // WB atlas is adopted in N.6+ and batches reference a shared atlas layer.
            uint texLayer = 0;

            var key = new GroupKey(
                batch.IBO, batch.FirstIndex, (int)batch.BaseVertex,
                batch.IndexCount, texHandle, texLayer, translucency, batch.CullMode);

            if (!_groups.TryGetValue(key, out var grp))
            {
                grp = new InstanceGroup
                {
                    Ibo = batch.IBO,
                    FirstIndex = batch.FirstIndex,
                    BaseVertex = (int)batch.BaseVertex,
                    IndexCount = batch.IndexCount,
                    BindlessTextureHandle = texHandle,
                    TextureLayer = texLayer,
                    Translucency = translucency,
                    CullMode = batch.CullMode,
                };
                _groups[key] = grp;
            }
            grp.Matrices.Add(model);
            grp.Slots.Add(_currentEntitySlot); // Phase U.4 — parallel to Matrices
            collector?.Add(new CachedBatch(key, texHandle, restPose));
        }
    }

    private ulong ResolveTexture(WorldEntity entity, MeshRef meshRef, ObjectRenderBatch batch, ulong palHash)
    {
        uint surfaceId = batch.Key.SurfaceId;
        if (surfaceId == 0 || surfaceId == 0xFFFFFFFF) return 0;

        uint overrideOrigTex = 0;
        bool hasOrigTexOverride = meshRef.SurfaceOverrides is not null
            && meshRef.SurfaceOverrides.TryGetValue(surfaceId, out overrideOrigTex);
        uint? origTexOverride = hasOrigTexOverride ? overrideOrigTex : (uint?)null;

        if (entity.PaletteOverride is not null)
        {
            return _textures.GetOrUploadWithPaletteOverrideBindless(
                surfaceId, origTexOverride, entity.PaletteOverride, palHash);
        }
        else if (hasOrigTexOverride)
        {
            return _textures.GetOrUploadWithOrigTextureOverrideBindless(surfaceId, overrideOrigTex);
        }
        else
        {
            return _textures.GetOrUploadBindless(surfaceId);
        }
    }

    private static void WriteMatrix(float[] buf, int offset, in Matrix4x4 m)
    {
        buf[offset + 0]  = m.M11; buf[offset + 1]  = m.M12; buf[offset + 2]  = m.M13; buf[offset + 3]  = m.M14;
        buf[offset + 4]  = m.M21; buf[offset + 5]  = m.M22; buf[offset + 6]  = m.M23; buf[offset + 7]  = m.M24;
        buf[offset + 8]  = m.M31; buf[offset + 9]  = m.M32; buf[offset + 10] = m.M33; buf[offset + 11] = m.M34;
        buf[offset + 12] = m.M41; buf[offset + 13] = m.M42; buf[offset + 14] = m.M43; buf[offset + 15] = m.M44;
    }

    /// <summary>
    /// Entity-set membership test. Phase U.1 (2026-05-30): with the
    /// two-pipe partition deleted, the sole <see cref="EntitySet.All"/>
    /// member matches every entity. Retained as a seam for the unified
    /// pass to re-introduce partitioning.
    /// </summary>
    private static bool EntityMatchesSet(WorldEntity entity, EntitySet set) => true;

    internal static bool EntityPassesVisibleCellGate(
        WorldEntity entity,
        HashSet<uint>? visibleCellIds,
        EntitySet set)
    {
        if (visibleCellIds is null)
            return true;

        if (entity.ParentCellId.HasValue)
            return visibleCellIds.Contains(entity.ParentCellId.Value);

        if (IsShellScopedSet(set) && entity.IsBuildingShell)
        {
            return entity.BuildingShellAnchorCellId is uint anchorCellId
                && visibleCellIds.Contains(anchorCellId);
        }

        return true;
    }

    // Phase U.1 (2026-05-30): the shell-scoped sets (IndoorPass / BuildingShells)
    // were deleted with the two-pipe machinery. EntitySet.All is never shell-scoped.
    private static bool IsShellScopedSet(EntitySet set) => false;

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;
        _gl.DeleteBuffer(_instanceSsbo);
        _gl.DeleteBuffer(_batchSsbo);
        _gl.DeleteBuffer(_indirectBuffer);
        if (_clipSlotSsbo != 0) _gl.DeleteBuffer(_clipSlotSsbo);               // Phase U.3
        if (_fallbackClipRegionSsbo != 0) _gl.DeleteBuffer(_fallbackClipRegionSsbo); // Phase U.3
        if (_gpuQueriesInitialized)
        {
            for (int i = 0; i < GpuQueryRingDepth; i++)
            {
                _gl.DeleteQuery(_gpuQueryOpaque[i]);
                _gl.DeleteQuery(_gpuQueryTransparent[i]);
            }
        }
    }

    // ── Public types + helpers for BuildIndirectArrays (Task 9) ─────────────
    //
    // These are public so the pure-CPU unit tests in AcDream.Core.Tests can
    // exercise BuildIndirectArrays without needing a GL context.

    /// <summary>
    /// Stride in bytes of <c>DrawElementsIndirectCommand</c> in the indirect buffer.
    /// 5 × <c>uint</c> = 20 bytes. Tests and callers reference this symbolically
    /// rather than hard-coding <c>20</c> so a layout change produces a compile error.
    /// </summary>
    public const int DrawCommandStride = 20; // sizeof(DrawElementsIndirectCommand): 5 × uint

    /// <summary>
    /// Public view of the per-group inputs to <see cref="BuildIndirectArrays"/> — used in tests.
    /// </summary>
    public readonly record struct IndirectGroupInput(
        int IndexCount,
        uint FirstIndex,
        int BaseVertex,
        int InstanceCount,
        int FirstInstance,
        ulong TextureHandle,
        uint TextureLayer,
        TranslucencyKind Translucency,
        CullMode CullMode = CullMode.CounterClockwise);

    /// <summary>
    /// Public mirror of the per-group <see cref="BatchData"/> uploaded to the SSBO.
    /// Tests verify the layout. Same field shape as the private BatchData.
    /// </summary>
    [StructLayout(LayoutKind.Sequential, Pack = 8)]
    public struct BatchDataPublic
    {
        public ulong TextureHandle;
        public uint  TextureLayer;
        public uint  Flags;
    }

    /// <summary>Result of <see cref="BuildIndirectArrays"/>.</summary>
    public readonly record struct IndirectLayoutResult(
        int OpaqueCount,
        int TransparentCount,
        int TransparentByteOffset);

    /// <summary>
    /// Lays out the indirect commands + parallel BatchData array contiguously:
    /// opaque section first (caller sorts before calling), transparent section second.
    /// Pure CPU, no GL state. Caller passes pre-sized scratch arrays.
    /// </summary>
    /// <remarks>
    /// Classification: Opaque + ClipMap → opaque pass (ClipMap uses discard, not
    /// blending). Everything else (AlphaBlend, Additive, InvAlpha) → transparent pass.
    /// </remarks>
    public static IndirectLayoutResult BuildIndirectArrays(
        IReadOnlyList<IndirectGroupInput> groups,
        DrawElementsIndirectCommand[] indirectScratch,
        BatchDataPublic[] batchScratch,
        CullMode[]? cullScratch = null)
    {
        int opaqueCount = 0;
        int transparentCount = 0;

        foreach (var g in groups)
        {
            if (IsOpaque(g.Translucency)) opaqueCount++;
            else transparentCount++;
        }

        int oi = 0;        // opaque write cursor (fills [0..opaqueCount))
        int ti = opaqueCount; // transparent write cursor (fills [opaqueCount..end))

        foreach (var g in groups)
        {
            var dec = new DrawElementsIndirectCommand
            {
                Count         = (uint)g.IndexCount,
                InstanceCount = (uint)g.InstanceCount,
                FirstIndex    = g.FirstIndex,
                BaseVertex    = g.BaseVertex,
                BaseInstance  = (uint)g.FirstInstance,
            };
            var bd = new BatchDataPublic
            {
                TextureHandle = g.TextureHandle,
                TextureLayer  = g.TextureLayer,
                Flags         = 0,
            };

            if (IsOpaque(g.Translucency))
            {
                indirectScratch[oi] = dec;
                batchScratch[oi]    = bd;
                if (cullScratch is not null) cullScratch[oi] = g.CullMode;
                oi++;
            }
            else
            {
                indirectScratch[ti] = dec;
                batchScratch[ti]    = bd;
                if (cullScratch is not null) cullScratch[ti] = g.CullMode;
                ti++;
            }
        }

        return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * DrawCommandStride);
    }

    /// <summary>
    /// Public test shim for <see cref="IsOpaque"/>. Locks in the N.5 Decision 2
    /// translucency partition: Opaque + ClipMap → opaque indirect; AlphaBlend +
    /// Additive + InvAlpha → transparent indirect.
    /// </summary>
    public static bool IsOpaquePublic(TranslucencyKind t) => IsOpaque(t);

    private static bool IsOpaque(TranslucencyKind t)
        => t == TranslucencyKind.Opaque || t == TranslucencyKind.ClipMap;

    // ────────────────────────────────────────────────────────────────────────

    /// <summary>
    /// Thin wrapper around an instance's rate-limit dictionary + frame
    /// counter, passed into the static <see cref="WalkEntitiesInto"/>
    /// overload so it can emit rate-limited probe lines without access
    /// to instance fields. Null = probes disabled (test-friendly overload).
    /// </summary>
    internal sealed class IndoorProbeState
    {
        private readonly Dictionary<ulong, int> _lastFrame;
        private readonly int _currentFrame;
        private const int RateLimit = IndoorProbeRateLimitFrames;

        internal IndoorProbeState(Dictionary<ulong, int> lastFrame, int currentFrame)
        {
            _lastFrame    = lastFrame;
            _currentFrame = currentFrame;
        }

        /// <summary>
        /// Returns true at most once per <see cref="IndoorProbeRateLimitFrames"/>
        /// frames per <paramref name="cellId"/>. Side-effect: stamps the frame
        /// number into the dictionary on success.
        /// </summary>
        internal bool ShouldEmit(ulong cellId)
        {
            if (!_lastFrame.TryGetValue(cellId, out int last)
                || _currentFrame - last >= RateLimit)
            {
                _lastFrame[cellId] = _currentFrame;
                return true;
            }
            return false;
        }
    }

    private sealed class InstanceGroup
    {
        public uint Ibo;
        public uint FirstIndex;
        public int BaseVertex;
        public int IndexCount;
        public ulong BindlessTextureHandle;   // 64-bit (was uint TextureHandle in N.4)
        public uint TextureLayer;             // 0 for per-instance composites; non-zero when WB atlas is adopted in N.6+
        public TranslucencyKind Translucency;
        public CullMode CullMode;
        public int FirstInstance;   // offset into the shared instance VBO (in instances, not bytes)
        public int InstanceCount;
        public float SortDistance;  // squared distance from camera to first instance, for opaque sort
        public readonly List<Matrix4x4> Matrices = new();

        // Phase U.4: per-instance clip-slot index, parallel to Matrices (Slots[i]
        // is the binding=2 CellClip slot for the instance whose matrix is
        // Matrices[i]). At layout time the dispatcher writes Slots[i] into
        // _clipSlotData at the same cursor it writes Matrices[i] into _instanceData,
        // so the binding=3 instanceClipSlot[] tracks the binding=0 instance.
        public readonly List<uint> Slots = new();
    }
}