feat(physics): Phase 2 — BuildingPhysics + CheckBuildingTransit

Closes the outdoor→indoor entry path. New BuildingPhysics type holds
the per-SortCell BldPortal list + building world transform; PhysicsDataCache
caches it (CacheBuilding + GetBuilding); CellTransit.CheckBuildingTransit
tests each portal's destination cell via PointInsideCellBsp.

PhysicsEngine.ResolveCellId's outdoor branch now hooks CheckBuildingTransit
after the terrain-grid lookup: if the matched landcell has a cached
building stab, check whether the sphere has crossed into one of its
interior EnvCells before returning.

GameWindow at landblock-load time iterates LandBlockInfo.Buildings and
caches each via PhysicsDataCache.CacheBuilding. The landcell-id derivation
uses retail's row-major cell-index formula (gridX * 8 + gridY + 1).

Polish items from Subagent B/C reviews folded in:
- visited HashSet in FindCellList's BFS (avoids O(N^2) re-enqueue)
- ResolveCellId_NoDataCache_ReturnsFallback test (closes coverage gap)
- DataCache-asymmetry comment in PhysicsEngine.ResolveCellId
- Replaced misleading FindCellList outdoor-branch TODO with explicit
  note that ResolveCellId bypasses this branch — wired in ResolveCellId
  directly.
- Removed unused 'using DatReaderWriter.Types;' from CellTransit.cs
- 2 new CellTransitFindCellListTests integration tests
- 1 new CellTransitCheckBuildingTransitTests test (null-CellBSP guard
  case; happy path deferred to visual verification).

Spec: docs/superpowers/specs/2026-05-19-indoor-portal-cell-tracking-design.md
Plan: docs/superpowers/plans/2026-05-19-indoor-portal-cell-tracking.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -5710,6 +5710,52 @@ public sealed class GameWindow : IDisposable
                 }
             }
 
+            // Phase 2: cache building portal lists for CellTransit.CheckBuildingTransit.
+            // Iterates LandBlockInfo.Buildings — each BuildingInfo has a Frame (world-
+            // relative origin + orientation) and a Portals list. The landcell id is
+            // derived from the building's frame origin using retail's row-major grid
+            // formula (gridX * 8 + gridY + 1) within the 192m × 192m landblock.
+            if (lbInfo is not null && lbInfo.Buildings.Count > 0)
+            {
+                uint lbPrefix = lb.LandblockId & 0xFFFF0000u;
+                foreach (var building in lbInfo.Buildings)
+                {
+                    if (building.Portals.Count == 0) continue;
+
+                    var bldPortals = new System.Collections.Generic.List<AcDream.Core.Physics.BldPortalInfo>(
+                        building.Portals.Count);
+                    foreach (var bp in building.Portals)
+                    {
+                        bldPortals.Add(new AcDream.Core.Physics.BldPortalInfo(
+                            otherCellId:   lbPrefix | (uint)bp.OtherCellId,
+                            otherPortalId: bp.OtherPortalId,
+                            flags:         (ushort)bp.Flags));
+                    }
+
+                    // Build a world transform for the building. Frame.Origin is
+                    // landblock-relative; add the landblock world origin to get
+                    // world space.
+                    var bldOriginWorld = building.Frame.Origin + origin;
+                    var buildingTransform =
+                        System.Numerics.Matrix4x4.CreateFromQuaternion(building.Frame.Orientation)
+                        * System.Numerics.Matrix4x4.CreateTranslation(bldOriginWorld);
+
+                    // Derive the outdoor landcell id containing this building.
+                    // Retail's cell index: row-major (gridX * 8 + gridY + 1) within
+                    // the 8×8 grid of 24m cells in a landblock.
+                    int bldGridX = (int)(building.Frame.Origin.X / 24f);
+                    int bldGridY = (int)(building.Frame.Origin.Y / 24f);
+                    if (bldGridX < 0) bldGridX = 0;
+                    if (bldGridX >= 8) bldGridX = 7;
+                    if (bldGridY < 0) bldGridY = 0;
+                    if (bldGridY >= 8) bldGridY = 7;
+                    uint landcellLow = (uint)(bldGridX * 8 + bldGridY + 1);
+                    uint landcellId = lbPrefix | landcellLow;
+
+                    _physicsDataCache.CacheBuilding(landcellId, bldPortals, buildingTransform);
+                }
+            }
+
             _physicsEngine.AddLandblock(lb.LandblockId, terrainSurface, cellSurfaces,
                 portalPlanes, origin.X, origin.Y);
         }

src/AcDream.Core/Physics/BuildingPhysics.cs

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Indoor walking Phase 2 (2026-05-19). Cached building portal data
+/// for outdoor→indoor cell entry. One per outdoor landcell that contains
+/// a building stab. Mirrors retail's <c>BuildingObj.Portals</c> array
+/// (per the pseudocode doc §"LandCell.find_transit_cells").
+/// </summary>
+public sealed class BuildingPhysics
+{
+    public required Matrix4x4 WorldTransform { get; init; }
+    public required Matrix4x4 InverseWorldTransform { get; init; }
+    public required IReadOnlyList<BldPortalInfo> Portals { get; init; }
+}
+
+/// <summary>
+/// One building portal: the connection from a SortCell's BuildingObj to
+/// an interior EnvCell. ExactMatch is decoded from <see cref="Flags"/>
+/// bit 0 (<c>PortalFlags.ExactMatch = 0x0001</c>).
+/// </summary>
+public readonly struct BldPortalInfo
+{
+    public BldPortalInfo(uint otherCellId, ushort otherPortalId, ushort flags)
+    {
+        OtherCellId   = otherCellId;
+        OtherPortalId = otherPortalId;
+        Flags         = flags;
+    }
+
+    /// <summary>Full id of the interior EnvCell this portal connects to.</summary>
+    public uint   OtherCellId   { get; }
+    /// <summary>The portal id within the destination EnvCell.</summary>
+    public ushort OtherPortalId { get; }
+    public ushort Flags         { get; }
+
+    /// <summary>Bit 0 of Flags (<c>PortalFlags.ExactMatch = 0x0001</c>).</summary>
+    public bool ExactMatch => (Flags & 0x0001) != 0;
+}

src/AcDream.Core/Physics/CellTransit.cs

@@ -1,6 +1,5 @@
 using System.Collections.Generic;
 using System.Numerics;
-using DatReaderWriter.Types;
 
 namespace AcDream.Core.Physics;
 
@@ -151,6 +150,36 @@ public static class CellTransit
         candidates.Add(lbPrefix | low);
     }
 
+    /// <summary>
+    /// Outdoor→indoor entry path. Ported from retail's
+    /// <c>BuildingObj::find_building_transit_cells</c> +
+    /// <c>EnvCell::check_building_transit</c>. For each portal of the
+    /// outdoor building, look up the destination interior cell and test
+    /// whether the sphere center is inside it via
+    /// <see cref="BSPQuery.PointInsideCellBsp"/>. If so, add the interior
+    /// cell to <paramref name="candidates"/>.
+    /// </summary>
+    public static void CheckBuildingTransit(
+        PhysicsDataCache cache,
+        BuildingPhysics building,
+        Vector3 worldSphereCenter,
+        float sphereRadius,
+        HashSet<uint> candidates)
+    {
+        foreach (var portal in building.Portals)
+        {
+            var otherCell = cache.GetCellStruct(portal.OtherCellId);
+            if (otherCell?.CellBSP?.Root is null) continue;
+
+            // Sphere center in the OTHER cell's local space.
+            var localCenter = Vector3.Transform(worldSphereCenter, otherCell.InverseWorldTransform);
+            if (BSPQuery.PointInsideCellBsp(otherCell.CellBSP.Root, localCenter))
+            {
+                candidates.Add(portal.OtherCellId);
+            }
+        }
+    }
+
     /// <summary>
     /// Top-level cell-tracking driver, ported from retail's
     /// <c>CObjCell::find_cell_list</c> (sphere variant).
@@ -188,7 +217,9 @@ public static class CellTransit
 
             // BFS the portal graph (one hop per pass — usually 1-2 passes is enough).
             var pending = new Queue<uint>();
+            var visited = new HashSet<uint>();
             pending.Enqueue(currentCellId);
+            visited.Add(currentCellId);
             int maxIterations = 16;  // hard cap; portal graphs are small
             while (pending.Count > 0 && maxIterations-- > 0)
             {
@@ -203,10 +234,9 @@ public static class CellTransit
 
                 if (candidates.Count > sizeBefore)
                 {
-                    // Snapshot the new candidates to avoid mutating during iteration.
                     foreach (var c in candidates)
                     {
-                        if (c != cellId)  // skip seed
+                        if (visited.Add(c))  // only enqueue if NEW
                             pending.Enqueue(c);
                     }
                 }
@@ -220,9 +250,28 @@ public static class CellTransit
         }
         else
         {
-            // Outdoor seed.
+            // Outdoor seed: expand neighbour landcells AND check for building stabs
+            // with portals into interior EnvCells.
             AddAllOutsideCells(worldSphereCenter, sphereRadius, currentCellId, candidates);
-            // Outdoor→indoor entry (CheckBuildingTransit) wires in a follow-up commit.
+
+            // For each landcell candidate, see if it carries a building stab; if so,
+            // check whether the sphere has crossed into any of the building's interior
+            // EnvCells via CheckBuildingTransit.
+            //
+            // NOTE: PhysicsEngine.ResolveCellId currently bypasses this entire branch
+            // for outdoor seeds (it uses its own _landblocks terrain grid loop). The
+            // outdoor→indoor production path therefore runs through ResolveCellId's
+            // OWN outdoor branch (see below for the call there too). This block is
+            // exercised by direct-FindCellList callers (tests, future re-entry from
+            // an indoor cell exiting through a portal that lands outside near a
+            // building).
+            var landcellSnapshot = new List<uint>(candidates);
+            foreach (uint landcellId in landcellSnapshot)
+            {
+                var building = cache.GetBuilding(landcellId);
+                if (building is null) continue;
+                CheckBuildingTransit(cache, building, worldSphereCenter, sphereRadius, candidates);
+            }
         }
 
         // Containment test: for each candidate, transform worldSphereCenter to

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -20,6 +20,9 @@ public sealed class PhysicsDataCache
     private readonly ConcurrentDictionary<uint, SetupPhysics> _setup = new();
     private readonly ConcurrentDictionary<uint, CellPhysics> _cellStruct = new();
 
+    // ── Phase 2: building portal cache for outdoor→indoor entry ───────────
+    private readonly ConcurrentDictionary<uint, BuildingPhysics> _buildings = new();
+
     /// <summary>
     /// Extract and cache the physics BSP + polygon data from a GfxObj,
     /// PLUS always cache a visual AABB from the vertex data regardless of
@@ -304,6 +307,31 @@ public sealed class PhysicsDataCache
     /// </summary>
     public void RegisterCellStructForTest(uint envCellId, CellPhysics physics)
         => _cellStruct[envCellId] = physics;
+
+    /// <summary>
+    /// Indoor walking Phase 2 (2026-05-19). Cache the building portal list
+    /// for an outdoor landcell that contains a building stab. Used by
+    /// <see cref="CellTransit.CheckBuildingTransit"/>.
+    /// </summary>
+    public void CacheBuilding(uint landcellId, IReadOnlyList<BldPortalInfo> portals, Matrix4x4 worldTransform)
+    {
+        if (_buildings.ContainsKey(landcellId)) return;
+        Matrix4x4.Invert(worldTransform, out var inverse);
+        _buildings[landcellId] = new BuildingPhysics
+        {
+            WorldTransform = worldTransform,
+            InverseWorldTransform = inverse,
+            Portals = portals,
+        };
+    }
+
+    public BuildingPhysics? GetBuilding(uint landcellId)
+        => _buildings.TryGetValue(landcellId, out var b) ? b : null;
+
+    public IReadOnlyCollection<uint> BuildingIds => (IReadOnlyCollection<uint>)_buildings.Keys;
+
+    /// <summary>Test helper, mirrors <see cref="RegisterCellStructForTest"/>.</summary>
+    public void RegisterBuildingForTest(uint landcellId, BuildingPhysics b) => _buildings[landcellId] = b;
 }
 
 /// <summary>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -258,7 +258,8 @@ public sealed class PhysicsEngine
 
         if (fallbackLow >= 0x0100u)
         {
-            // Indoor seed: use portal-graph traversal.
+            // Indoor branch needs DataCache to look up cells; outdoor uses
+            // _landblocks (no DataCache dependency).
             if (DataCache is null) return fallbackCellId;
             return CellTransit.FindCellList(DataCache, worldPos, sphereRadius, fallbackCellId);
         }
@@ -274,7 +275,29 @@ public sealed class PhysicsEngine
             if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
             {
                 uint lowCellId = lb.Terrain.ComputeOutdoorCellId(localX, localY);
-                return (kvp.Key & 0xFFFF0000u) | lowCellId;
+                uint outdoorCellId = (kvp.Key & 0xFFFF0000u) | lowCellId;
+
+                // Outdoor→indoor entry: if this landcell has a cached building,
+                // check whether the sphere has crossed into one of its interior
+                // EnvCells via the building's portals.
+                if (DataCache is not null)
+                {
+                    var building = DataCache.GetBuilding(outdoorCellId);
+                    if (building is not null)
+                    {
+                        var candidates = new System.Collections.Generic.HashSet<uint>();
+                        CellTransit.CheckBuildingTransit(
+                            DataCache, building, worldPos, sphereRadius, candidates);
+                        if (candidates.Count > 0)
+                        {
+                            // First candidate wins — building portal containment is
+                            // mutually exclusive in retail (one interior cell per portal).
+                            foreach (var c in candidates) return c;
+                        }
+                    }
+                }
+
+                return outdoorCellId;
             }
         }