fix #108-residual (root cause): terrain drew DOUBLE-SIDED - port retail landPolysDraw eye-side gate as terrain backface cull

The cellar-ascent grass window was the UNDERSIDE of the z~94 grade
sheet. Retail terrain is single-sided: ACRender::landPolysDraw
(0x006b7040) draws each land triangle ONLY when the camera is on the
POSITIVE (upper) side of its plane (Plane::which_side2 vs
Render::FrameCurrent, zFightTerrainAdjust bias) - a below-grade eye
gets NO terrain, so retail shows sky through the cellar door.

We inherited WB's frame-global cull DISABLE (WB GameScene.cs:841 - an
editor camera goes underground by design) and TerrainModernRenderer.Draw
set no cull state of its own -> terrain rasterized both sides. From a
below-grade eye every aperture sight-ray RISES, so the only 'terrain'
it can see is the grade sheet's underside - which painted the exit-door
aperture (the landscape slice's 2D NDC clip planes (nx,ny,0,dw) have no
depth axis and cannot exclude between-eye-and-portal geometry) and slid
off the door exactly as the eye crossed grade. Membership/viewer was
exonerated by the harness in the previous commit.

Fix: TerrainModernRenderer.Draw owns its cull state (the 7th
self-contained-GL-state instance): Enable(CullFace) + CullFace(Back) +
FrontFace(Ccw), set -> draw -> restore the frame-global CW + cull-off
baseline. GL backface culling evaluates retail's per-triangle eye-side
predicate at rasterization; no shader change.

Pins:
- LandblockMeshTests.Build_AllTriangles_WindCounterClockwiseInWorldXY:
  every emitted triangle CCW in world XY across both FSplitNESW split
  directions - the winding invariant culling depends on.
- TerrainCullOrientationTests: under the production camera convention
  (LookAt up=+Z, Numerics perspective) an up-facing triangle winds CCW
  in window space from above (kept) and CW from below (culled) - guards
  FrontFace inversion, which would blank terrain from above.

Oracle note: retail's through-portal clip has NO portal-face near plane
(PView::GetClip / Render::set_view install edge planes only); nearer-
than-portal exclusion comes from the eye-side cull + cell-level
admission. No register row: this PORTS the retail mechanism, retiring
an undocumented WB-heritage deviation.

Gate pending: cellar climb (grass window gone) + outdoor sanity glance
(terrain intact from above).

Suites: App 263+1skip / Core 1443+2skip / UI 420 / Net 294.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ISSUES.md

@@ -3701,27 +3701,47 @@ Unverified. The likely culprits, ranked by suspected probability:
 
 ---
 
-## #108 — Cellar↔main-floor transition: terrain (grass) sweeps across the upstairs door opening — [REOPENED 2026-06-11 · narrowed residual]
+## #108 — Cellar↔main-floor transition: terrain (grass) sweeps across the upstairs door opening — [FIX SHIPPED 2026-06-12 · awaiting cellar visual gate]
 
-**Status:** REOPENED (narrowed) — the broad symptom is GONE (T5 +
-re-gate #2: "Yes, but…"), but a residual remains in ONE window: during
-the cellar ASCENT, while the eye is still below ground level, the
-upstairs exit-door opening is covered with grass — "like the ground
-level rose to the top of the door … as soon as my head pops up it falls
-back to ground level" (user, re-gate 2026-06-11). The original
-BR-2-era diagnosis stands: grass-sweep frames render through the
-OUTDOOR root (membership/viewer-cell flips outdoor mid-cellar), and the
-#117 depth-gated punch then correctly refuses to punch the aperture
-where terrain depth is NEARER than the door fan (eye below grade ⇒ the
-visible front-facing terrain can sit between the eye and the door in
-depth). The punch must STAY depth-gated (DO-NOT-RETRY) — the fix is on
-the membership/viewer side (why is the root outdoor while the eye is in
-the cellar stairwell below grade?). Apparatus shape: a vertical
-cellar-ascent variant of the #118 exit-walk harness (drive the eye up
-the stair path; log root resolution + the punch's mark-pass outcome per
-step). Prior history below.
+**Status:** FIX SHIPPED (desk-pinned) — root cause found 2026-06-12; the
+cellar-ascent visual gate is pending.
+
+**ROOT CAUSE (2026-06-12): terrain was drawn DOUBLE-SIDED — the grass was
+the UNDERSIDE of the grade sheet.** Two steps:
+1. The membership/viewer re-diagnosis below is **REFUTED** by the vertical
+   cellar-ascent harness (`Issue108CellarAscentViewerReplayTests`, dat-backed
+   A9B4 corner-building cellar 0x0174→0x0175→0x0171, production
+   FindCellList pick + the camera probe chain mirrored verbatim): 0
+   outdoor/null viewer resolutions while the eye is below grade, 0 sweep
+   failures, 0 fallback branches across boom distance {2.61, 5} × damping
+   lag {0, 0.3}. The viewer enters 0x0171 at eye z 94.01 — exactly as the
+   head pops above grade (the stairwell portal sits at grade), matching the
+   user's wording. The root is INTERIOR the whole window.
+2. Retail terrain is SINGLE-SIDED: `ACRender::landPolysDraw` (0x006b7040)
+   draws each land triangle ONLY when the camera is on the POSITIVE (upper)
+   side of its plane (`Plane::which_side2` vs `Render::FrameCurrent`). A
+   below-grade eye gets NO terrain — through the door retail shows sky.
+   WB renders the world with face culling DISABLED frame-globally (WB
+   `GameScene.cs:841` — editor heritage), and `TerrainModernRenderer.Draw`
+   set no cull state of its own → terrain drew double-sided. From a
+   below-grade eye every aperture sight-ray RISES, so the only "terrain" it
+   can see is the underside of the z≈94 grade sheet — which painted the
+   whole exit-door aperture (the landscape slice's 2D NDC clip planes
+   `(nx,ny,0,dw)` have no depth axis and cannot exclude it) and slid down
+   off the door exactly as the eye crossed grade.
+   **Fix: port the landPolysDraw eye-side gate as terrain backface culling**
+   — `TerrainModernRenderer.Draw` now owns Enable(CullFace) + Cull(Back) +
+   FrontFace(Ccw) (set→draw→restore; 7th instance of the self-contained-GL-
+   state rule). Pins: `LandblockMeshTests.Build_AllTriangles_WindCounter-
+   ClockwiseInWorldXY` (every emitted triangle CCW in world XY — cull-safe
+   winding) + `TerrainCullOrientationTests` (above-eye ⇒ CCW window winding
+   kept / below-eye ⇒ CW culled under the production camera convention).
+**Gate:** climb out of the corner-building cellar — the grass window over
+the exit door must be gone (sky/world through the door instead); plus a
+general outdoor sanity glance (terrain intact from above — a wrong
+FrontFace would blank it).
 **Severity:** MEDIUM
-**Component:** ~~render / indoor PView~~ → **physics / membership** (cellar-transition root flip)
+**Component:** render / terrain (single-sidedness) — membership/viewer EXONERATED
 
 During the cellar→main-floor ascent (Holtburg), the door opening visible on the main floor
 shows the outdoor GRASS texture sweeping over it — "like outdoor ground rising up from the

src/AcDream.App/Rendering/TerrainModernRenderer.cs

@@ -283,6 +283,27 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
         // when wired, else the no-clip fallback (count 0 = ungated terrain).
         BindClipUboBinding2();
 
+        // #108-residual: retail terrain is SINGLE-SIDED — ACRender::landPolysDraw
+        // (0x006b7040) draws each land triangle ONLY when the camera is on the
+        // POSITIVE (upper) side of its plane (Plane::which_side2 vs
+        // Render::FrameCurrent, zFightTerrainAdjust bias). GL backface culling
+        // evaluates the same per-triangle eye-side predicate at rasterization.
+        // LandblockMesh emits every triangle CCW in world XY seen from above
+        // (LandblockMeshTests winding pin), which the unified camera chain
+        // (CreateLookAt up=+Z + Numerics perspective) maps to CCW window
+        // winding from above / CW from below (TerrainCullOrientationTests) —
+        // so FrontFace(Ccw)+Cull(Back) keeps the top side and culls the
+        // underside. WB drew the whole world with culling DISABLED
+        // frame-globally (WB GameScene.cs:841 — an editor camera goes
+        // underground); inheriting that drew terrain DOUBLE-SIDED, and a
+        // below-grade eye (cellar ascent) saw the UNDERSIDE of the grade
+        // sheet through the exit-door aperture — the #108 grass window.
+        // Self-contained state per feedback_render_self_contained_gl_state;
+        // the frame-global CW + cull-off baseline is restored after the draw.
+        _gl.Enable(EnableCap.CullFace);
+        _gl.CullFace(TriangleFace.Back);
+        _gl.FrontFace(FrontFaceDirection.Ccw);
+
         _gl.BindVertexArray(_globalVao);
         _gl.MemoryBarrier(MemoryBarrierMask.CommandBarrierBit);
         _gl.MultiDrawElementsIndirect(
@@ -292,6 +313,9 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
             (uint)sizeof(DrawElementsIndirectCommand));
         _gl.BindVertexArray(0);
         _gl.BindBuffer(GLEnum.DrawIndirectBuffer, 0);
+
+        _gl.FrontFace(FrontFaceDirection.CW);
+        _gl.Disable(EnableCap.CullFace);
     }
 
     public void Dispose()

tests/AcDream.App.Tests/Rendering/TerrainCullOrientationTests.cs

@@ -0,0 +1,82 @@
+using System;
+using System.Numerics;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+/// <summary>
+/// #108-residual orientation pin: TerrainModernRenderer culls terrain back
+/// faces with FrontFace(Ccw) — the GL port of retail's single-sided terrain
+/// (ACRender::landPolysDraw 0x006b7040: a land triangle draws ONLY when the
+/// camera is on the POSITIVE side of its plane via Plane::which_side2).
+///
+/// The FrontFace choice rests on one mapping fact: under the production
+/// camera convention (Matrix4x4.CreateLookAt with up = world +Z, Numerics
+/// CreatePerspectiveFieldOfView — RetailChaseCamera.cs:203 / :52), an
+/// UP-FACING terrain triangle that LandblockMesh emits CCW in world XY
+/// rasterizes
+///   · CCW in NDC/window space when the eye is ABOVE its plane (kept), and
+///   · CW when the eye is BELOW (culled — retail draws nothing there: from
+///     a below-grade cellar eye the door aperture shows sky, never grass).
+/// This test pins that mapping in pure CPU math so a projection-convention
+/// change (handedness, Y-flip) can't silently invert the cull and either
+/// resurrect the #108 grass window or cull terrain from above.
+/// </summary>
+public class TerrainCullOrientationTests
+{
+    // An up-facing triangle, CCW in world XY viewed from above — the exact
+    // emission convention pinned by LandblockMeshTests (crossZ > 0).
+    private static readonly Vector3[] Triangle =
+    {
+        new(-1f, 10f, 94f),
+        new( 1f, 10f, 94f),
+        new( 1f, 12f, 94f),
+    };
+
+    private static float NdcSignedArea2(Vector3 eye, Vector3 forward)
+    {
+        // The production camera shape: look-at with world-Z up
+        // (RetailChaseCamera.cs:203), Numerics perspective with the retail
+        // znear 0.1 (RetailChaseCamera.cs:52).
+        var view = Matrix4x4.CreateLookAt(eye, eye + forward, Vector3.UnitZ);
+        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 0.1f, 5000f);
+        var viewProj = view * proj;
+
+        Span<Vector2> ndc = stackalloc Vector2[3];
+        for (int i = 0; i < 3; i++)
+        {
+            var c = Vector4.Transform(new Vector4(Triangle[i], 1f), viewProj);
+            Assert.True(c.W > 1e-3f, "test triangle must be in front of the eye");
+            ndc[i] = new Vector2(c.X / c.W, c.Y / c.W);
+        }
+
+        // Twice the signed area: > 0 = CCW in NDC (GL window space keeps the
+        // orientation — NDC y up maps to window y up, no flip).
+        return (ndc[1].X - ndc[0].X) * (ndc[2].Y - ndc[0].Y)
+             - (ndc[1].Y - ndc[0].Y) * (ndc[2].X - ndc[0].X);
+    }
+
+    [Fact]
+    public void EyeAboveTerrainPlane_WindsCcw_FrontFaceKept()
+    {
+        // Eye above grade looking forward-down at the triangle (the normal
+        // outdoor view). Retail: which_side2 = POSITIVE → drawn.
+        float area = NdcSignedArea2(new Vector3(0f, 5f, 96.5f), new Vector3(0f, 1f, -0.3f));
+        Assert.True(area > 0f,
+            $"above-plane eye must see the terrain triangle CCW (area2={area}) — " +
+            "FrontFace(Ccw)+Cull(Back) would otherwise cull terrain from above");
+    }
+
+    [Fact]
+    public void EyeBelowTerrainPlane_WindsCw_BackfaceCulled()
+    {
+        // Eye below grade (the cellar-stairwell window) looking up-forward at
+        // the underside. Retail: which_side2 = NEGATIVE → not drawn at all —
+        // the #108 grass that covered the exit door was exactly this
+        // underside rasterizing when culling was left disabled.
+        float area = NdcSignedArea2(new Vector3(0f, 5f, 92.5f), new Vector3(0f, 1f, 0.2f));
+        Assert.True(area < 0f,
+            $"below-plane eye must see the terrain triangle CW (area2={area}) — " +
+            "it must backface-cull like retail's which_side2 eye-side gate");
+    }
+}

tests/AcDream.Core.Tests/Physics/Issue108CellarAscentViewerReplayTests.cs

@@ -0,0 +1,344 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.Physics;
+using AcDream.Core.Tests.Conformance;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// #108-residual vertical exit-walk harness (2026-06-12): the cellar-ascent
+/// grass window. Climbing out of the Holtburg corner-building cellar
+/// (0xA9B40174 room, floor z≈90 → 0x0175 staircase/lip → 0x0171 main floor at
+/// z=94 = outdoor grade), the upstairs exit door is covered with grass until
+/// the eye pops above grade. Punch/seal are exonerated (BR-2 experiment +
+/// #117); the grass requires the frame to render through the OUTDOOR root —
+/// i.e. the VIEWER-CELL resolution demotes to outdoor/null while the eye is
+/// still below terrain grade inside the stairwell.
+///
+/// This harness drives the PRODUCTION viewer-resolution stack headlessly per
+/// step of a kinematic ascent (the #118 HouseExitWalkReplayTests pattern,
+/// turned vertical):
+///   player cell  — CellTransit.FindCellList on the foot-sphere center (the
+///                  production controller pick),
+///   viewer cell  — the PhysicsCameraCollisionProbe.SweepEye chain mirrored
+///                  verbatim (CameraCornerSealReplayTests provenance):
+///                  AdjustPosition at the head pivot → ResolveWithTransition
+///                  (IsViewer|PathClipped|FreeRotate|PerfectClip, 0.3 m
+///                  viewer_sphere) → fallback 1 AdjustPosition at the sought
+///                  eye → fallback 2 (player_pos, cell 0).
+/// Each step records WHICH branch produced the viewer cell, so a demote
+/// self-attributes:
+///   A. sweep Ok=false → fallback chain (AdjustPosition's SeenOutside
+///      fall-through is an XY-only grid snap — no Z test — so an in-dirt
+///      below-grade eye can return an OUTDOOR cell with found=true);
+///   B. sweep end-cell pick demotes (exterior-portal straddle + containment
+///      miss at the stopped eye);
+///   C. the start-cell AdjustPosition at the pivot demotes;
+///   D. all healthy here → the bug is upstream (App camera damping /
+///      GameWindow TryGetCell consumption).
+///
+/// Ascent path: fitted from the live captures (cellar-up-capture*.jsonl band
+/// centroids, analyze_108_stairline.py): stairs at x≈153.9 ascending +Y,
+/// z = 90.0 (y≤5.7) → 0.836·(y−5.73)+90.25 (stairs) → lip 93.25→94 over
+/// y 9.3→10.4 → main floor 94.0. The boom (retail defaults: distance 2.61,
+/// pitch 0.291, pivot feet+1.5) trails SOUTH into the stairwell — mid-stairs
+/// the desired eye sits beyond the cellar's south wall (y≈4.87) and above its
+/// ceiling: in no-cell dirt below grade. Stub terrain (−1000) — the membership
+/// pick never reads terrain height (XY-column only), which is exactly the
+/// mechanism under test.
+///
+/// ── RESULT (2026-06-12): the MEMBERSHIP/VIEWER LAYER IS EXONERATED ──────
+/// 0 grass-window steps, 0 sweep failures, 0 fallback branches across boom
+/// distance {2.61, 5.0} × damping lag {0, 0.3 m}. The viewer resolves
+/// 0x0174 → 0x0175 (eye z 93.65, below grade) → 0x0171 at eye z 94.01 —
+/// the viewer enters the main-floor room EXACTLY as the head pops above
+/// grade (the stairwell portal sits at grade), matching the user's wording.
+/// The handoff's "it is MEMBERSHIP/VIEWER-side" diagnosis is therefore
+/// REFUTED for the current pipeline; #108-residual is RENDER-side: the
+/// landscape slice clips terrain by 2D NDC planes only ((nx,ny,0,dw) —
+/// ClipFrame.cs:178, terrain_modern.vert:173), so terrain BETWEEN the eye
+/// and the exit portal (the grade sheet at z≈94, which from a below-grade
+/// eye projects into the aperture band at y 9.8–17) paints the doorway.
+/// These tests stay as the characterization pin for the healthy layer.
+/// </summary>
+public class Issue108CellarAscentViewerReplayTests
+{
+    private readonly ITestOutputHelper _out;
+    public Issue108CellarAscentViewerReplayTests(ITestOutputHelper output) => _out = output;
+
+    private const float ViewerSphereRadius = 0.3f;  // retail viewer_sphere (acclient :93314)
+    private const float PivotHeight = 1.5f;         // RetailChaseCamera.PivotHeight
+    private const float FootRadius = 0.48f;         // player foot sphere
+    private const float BoomDistance = 2.61f;       // retail viewer_offset length
+    private const float BoomPitch = 0.291f;         // retail default pitch (16.7°)
+    private const float GradeZ = 94.0f;             // cottage floor == door sill ≈ outdoor terrain grade
+
+    private const uint Lb = 0xA9B40000u;            // ConformanceDats.HoltburgLandblock
+    private const uint CellarRoom = Lb | 0x0174u;   // floor z≈90.0
+    private const uint MainFloor = Lb | 0x0171u;    // z=94.0
+
+    // ── fixture ─────────────────────────────────────────────────────────
+
+    private static (PhysicsEngine engine, PhysicsDataCache cache,
+        Dictionary<uint, AcDream.Core.World.Cells.EnvCell> envCells)
+        BuildEngine(DatCollection dats)
+    {
+        var cache = new PhysicsDataCache();
+        var engine = new PhysicsEngine { DataCache = cache };
+        var envCells = new Dictionary<uint, AcDream.Core.World.Cells.EnvCell>();
+
+        // Full A9B4 interior set (Issue112MembershipTests.LoadLandblockInteriors
+        // pattern) — the ascent's pick walk may reach cells outside the corner
+        // building's 0x016F-0x0175 range.
+        for (uint low = 0x0100u; low <= 0x01FFu; low++)
+        {
+            try { envCells[Lb | low] = ConformanceDats.LoadEnvCell(dats, cache, Lb | low); }
+            catch { }
+        }
+
+        // Buildings exactly as production registers them (Issue112MembershipTests.
+        // RegisterBuildings provenance): portals → BldPortalInfo with sign-extended
+        // OtherPortalId; landcell id from the building Frame.Origin (retail
+        // row-major grid).
+        var lbInfo = dats.Get<DatReaderWriter.DBObjs.LandBlockInfo>(Lb | 0xFFFEu);
+        Assert.NotNull(lbInfo);
+        foreach (var building in lbInfo!.Buildings)
+        {
+            if (building.Portals.Count == 0) continue;
+            var portals = new List<BldPortalInfo>(building.Portals.Count);
+            foreach (var bp in building.Portals)
+                portals.Add(new BldPortalInfo(
+                    otherCellId: Lb | (uint)bp.OtherCellId,
+                    otherPortalId: unchecked((short)bp.OtherPortalId),
+                    flags: (ushort)bp.Flags));
+            var transform =
+                Matrix4x4.CreateFromQuaternion(building.Frame.Orientation) *
+                Matrix4x4.CreateTranslation(building.Frame.Origin);
+            int gridX = (int)(building.Frame.Origin.X / 24f);
+            int gridY = (int)(building.Frame.Origin.Y / 24f);
+            uint landcellLow = (uint)(gridX * 8 + gridY + 1);
+            cache.CacheBuilding(Lb | landcellLow, portals, transform);
+        }
+
+        var heights = new byte[81];
+        var heightTable = new float[256];
+        for (int i = 0; i < 256; i++) heightTable[i] = -1000f;
+        engine.AddLandblock(Lb, new TerrainSurface(heights, heightTable),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(), 0f, 0f);
+
+        return (engine, cache, envCells);
+    }
+
+    // ── the probe mirror (PhysicsCameraCollisionProbe.SweepEye, verbatim) ──
+
+    private enum ViewerBranch { Sweep, AdjustFallback, NullFallback }
+
+    private sealed record ViewerResolve(
+        Vector3 Eye, uint ViewerCellId, ViewerBranch Branch,
+        uint StartCell, bool PivotAdjustFound, ResolveResult Sweep);
+
+    private static ViewerResolve ResolveViewer(
+        PhysicsEngine engine, Vector3 pivot, Vector3 desiredEye, uint cellId, Vector3 playerPos)
+    {
+        // update_viewer (pc:92775): no player cell → snap to player, viewer_cell null.
+        if (cellId == 0u)
+            return new ViewerResolve(playerPos, 0u, ViewerBranch.NullFallback, 0u, false, default);
+
+        uint startCell = cellId;
+        bool pivotFound = false;
+        if ((cellId & 0xFFFFu) >= 0x0100u)
+        {
+            var (pivotCell, found) = engine.AdjustPosition(cellId, pivot);
+            pivotFound = found;
+            if (found) startCell = pivotCell;
+        }
+
+        Vector3 begin = pivot - new Vector3(0f, 0f, ViewerSphereRadius);
+        Vector3 end = desiredEye - new Vector3(0f, 0f, ViewerSphereRadius);
+
+        var r = engine.ResolveWithTransition(
+            currentPos: begin,
+            targetPos: end,
+            cellId: startCell,
+            sphereRadius: ViewerSphereRadius,
+            sphereHeight: 0f,
+            stepUpHeight: 0f,
+            stepDownHeight: 0f,
+            isOnGround: false,
+            body: null,
+            moverFlags: ObjectInfoState.IsViewer | ObjectInfoState.PathClipped
+                      | ObjectInfoState.FreeRotate | ObjectInfoState.PerfectClip,
+            movingEntityId: 0);
+
+        Vector3 eye = r.Position + new Vector3(0f, 0f, ViewerSphereRadius);
+        if (r.Ok)
+            return new ViewerResolve(eye, r.CellId, ViewerBranch.Sweep, startCell, pivotFound, r);
+
+        var (eyeCell, eyeFound) = engine.AdjustPosition(cellId, desiredEye);
+        if (eyeFound)
+            return new ViewerResolve(desiredEye, eyeCell, ViewerBranch.AdjustFallback, startCell, pivotFound, r);
+
+        return new ViewerResolve(playerPos, 0u, ViewerBranch.NullFallback, startCell, pivotFound, r);
+    }
+
+    // ── the ascent ──────────────────────────────────────────────────────
+
+    /// <summary>Stair-line feet Z for a path y (fitted from the capture bands).</summary>
+    private static float FeetZ(float y)
+    {
+        if (y < 5.73f) return 90.0f;
+        if (y < 9.30f) return MathF.Min(90.25f + 0.836f * (y - 5.73f), 93.25f);
+        if (y < 10.40f) return 93.25f + (y - 9.30f) * (0.75f / 1.10f);
+        return 94.0f;
+    }
+
+    private sealed record Step(
+        int Index, Vector3 Feet, uint PlayerCell,
+        ViewerResolve Viewer, uint EyeContainedIn, bool EyeBelowGrade)
+    {
+        public bool ViewerOutdoorOrNull =>
+            Viewer.ViewerCellId == 0u || (Viewer.ViewerCellId & 0xFFFFu) < 0x0100u;
+    }
+
+    private List<Step>? RunAscent(float boomDistance, float pathLagMeters)
+    {
+        var datDir = ConformanceDats.ResolveDatDir();
+        if (datDir is null) { _out.WriteLine("SKIP: dats unavailable"); return null; }
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var (engine, _, envCells) = BuildEngine(dats);
+
+        const float yStart = 5.2f, yEnd = 16.0f;
+        const float stepLen = 0.02f;                 // 2 cm/frame ≈ 1.2 m/s at 60 Hz
+        var fwd = new Vector3(0f, 1f, 0f);           // facing up the stairs / at the exit door
+        float cosP = MathF.Cos(BoomPitch), sinP = MathF.Sin(BoomPitch);
+
+        // Stairs run at x≈153.9; past the lip the real walk line bends to the
+        // exit-door approach at x≈155 (corner-seal capture S1: player
+        // (154.93, 16.45)) — walking straight north at 153.9 ends in the wall
+        // beside the 0x0170 doorway, which a live player cannot do.
+        static float FeetX(float y) =>
+            y <= 10.4f ? 153.9f
+            : y >= 14.0f ? 155.0f
+            : 153.9f + (y - 10.4f) / (14.0f - 10.4f) * (155.0f - 153.9f);
+
+        var steps = new List<Step>();
+        uint playerCell = CellarRoom;
+        int count = (int)MathF.Round((yEnd - yStart) / stepLen);
+
+        for (int i = 0; i <= count; i++)
+        {
+            float y = yStart + i * stepLen;
+            var feet = new Vector3(FeetX(y), y, FeetZ(y));
+
+            // production controller pick: foot-sphere CENTER, seeded with the carried cell
+            playerCell = CellTransit.FindCellList(
+                engine.DataCache!, feet + new Vector3(0f, 0f, FootRadius), FootRadius, playerCell);
+
+            // boom target — optionally computed from a lagged path point to model the
+            // exponential damping trail (≈0.27 m at climb speed; 0 = converged target)
+            float yBoom = MathF.Max(yStart, y - pathLagMeters);
+            var boomFeet = new Vector3(FeetX(yBoom), yBoom, FeetZ(yBoom));
+            var pivot = feet + new Vector3(0f, 0f, PivotHeight);
+            var boomPivot = boomFeet + new Vector3(0f, 0f, PivotHeight);
+            var desiredEye = boomPivot - fwd * (boomDistance * cosP)
+                           + new Vector3(0f, 0f, boomDistance * sinP);
+
+            var viewer = ResolveViewer(engine, pivot, desiredEye, playerCell, feet);
+
+            uint containedIn = 0u;
+            foreach (var (id, env) in envCells)
+                if (env.PointInCell(viewer.Eye)) { containedIn = id; break; }
+
+            steps.Add(new Step(i, feet, playerCell, viewer,
+                containedIn, viewer.Eye.Z < GradeZ - 0.05f));
+        }
+
+        return steps;
+    }
+
+    private void DumpStep(Step s)
+    {
+        var v = s.Viewer;
+        string line = FormattableString.Invariant(
+            $"step={s.Index,3} feet=({s.Feet.X:F2},{s.Feet.Y:F2},{s.Feet.Z:F2}) pCell=0x{s.PlayerCell & 0xFFFFu:X4} start=0x{v.StartCell & 0xFFFFu:X4}{(v.PivotAdjustFound ? "" : "!")} branch={v.Branch} ok={v.Sweep.Ok} eye=({v.Eye.X:F2},{v.Eye.Y:F2},{v.Eye.Z:F2}) viewer=0x{v.ViewerCellId & 0xFFFFu:X4} eyeIn=0x{s.EyeContainedIn & 0xFFFFu:X4} belowGrade={(s.EyeBelowGrade ? "Y" : "n")}");
+        if (s.EyeBelowGrade && s.ViewerOutdoorOrNull) line += "  << GRASS-WINDOW";
+        _out.WriteLine(line);
+    }
+
+    // ── diagnostics + pins ──────────────────────────────────────────────
+
+    /// <summary>
+    /// Full per-step table of the ascent at retail boom defaults (converged
+    /// boom, no lag). Read this first — the GRASS-WINDOW marks name the steps
+    /// where the production stack resolves an outdoor/null viewer with the eye
+    /// below grade, and the branch column attributes the demote site.
+    /// </summary>
+    [Fact]
+    public void Diagnostic_CellarAscent_PerStepTable()
+    {
+        var steps = RunAscent(BoomDistance, pathLagMeters: 0f);
+        if (steps is null) return;
+
+        uint lastPlayer = 0; uint lastViewer = 0xFFFFFFFFu; var lastBranch = (ViewerBranch)(-1);
+        int suspicious = 0;
+        foreach (var s in steps)
+        {
+            bool grass = s.EyeBelowGrade && s.ViewerOutdoorOrNull;
+            if (grass) suspicious++;
+            if (s.PlayerCell != lastPlayer || s.Viewer.ViewerCellId != lastViewer
+                || s.Viewer.Branch != lastBranch || grass || s.Index % 50 == 0)
+                DumpStep(s);
+            lastPlayer = s.PlayerCell; lastViewer = s.Viewer.ViewerCellId; lastBranch = s.Viewer.Branch;
+        }
+        _out.WriteLine(FormattableString.Invariant(
+            $"--- {suspicious}/{steps.Count} steps in the grass window (viewer outdoor/null while eye below grade) ---"));
+    }
+
+    /// <summary>Boom-distance + damping-lag sweep: how wide is the window across poses?</summary>
+    [Fact]
+    public void Diagnostic_CellarAscent_PoseSweep()
+    {
+        foreach (float dist in new[] { 2.61f, 5.0f })
+            foreach (float lag in new[] { 0f, 0.30f })
+            {
+                var steps = RunAscent(dist, lag);
+                if (steps is null) return;
+                int grass = steps.FindAll(s => s.EyeBelowGrade && s.ViewerOutdoorOrNull).Count;
+                int okFalse = steps.FindAll(s => !s.Viewer.Sweep.Ok).Count;
+                int fb = steps.FindAll(s => s.Viewer.Branch != ViewerBranch.Sweep).Count;
+                _out.WriteLine(FormattableString.Invariant(
+                    $"dist={dist:F2} lag={lag:F2}: grassWindow={grass}/{steps.Count} sweepOkFalse={okFalse} fallbackBranch={fb}"));
+            }
+    }
+
+    /// <summary>
+    /// THE PIN: while the eye is below terrain grade on the cellar ascent, the
+    /// viewer must resolve INTERIOR — an outdoor/null viewer cell roots the
+    /// frame at the landscape and sweeps grass across the exit door (#108).
+    /// Retail's viewer rides the stairwell cells here (the cellar camera works
+    /// in retail); below grade inside the building footprint there is no
+    /// legitimate outdoor viewer.
+    /// </summary>
+    [Fact]
+    public void CellarAscent_ViewerStaysInterior_WhileEyeBelowGrade()
+    {
+        var steps = RunAscent(BoomDistance, pathLagMeters: 0f);
+        if (steps is null) return;
+
+        var failures = steps.FindAll(s => s.EyeBelowGrade && s.ViewerOutdoorOrNull);
+        if (failures.Count > 0)
+        {
+            _out.WriteLine($"--- {failures.Count} grass-window steps ---");
+            foreach (var s in failures) DumpStep(s);
+        }
+        Assert.True(failures.Count == 0,
+            $"{failures.Count}/{steps.Count} ascent steps resolve an outdoor/null viewer cell while the eye " +
+            "is below grade — the #108 grass window (see output for the branch attribution)");
+    }
+}

tests/AcDream.Core.Tests/Terrain/LandblockMeshTests.cs

@@ -169,6 +169,41 @@ public class LandblockMeshTests
         Assert.True(cache.Count >= 2, $"Expected mix of palette codes, got {cache.Count}");
     }
 
+    [Fact]
+    public void Build_AllTriangles_WindCounterClockwiseInWorldXY()
+    {
+        // #108-residual winding pin: TerrainModernRenderer enables backface
+        // culling with FrontFace(Ccw) — the GL port of retail's single-sided
+        // terrain (ACRender::landPolysDraw 0x006b7040 draws a land triangle
+        // only when the eye is on the POSITIVE side of its plane). That cull
+        // is only correct if EVERY emitted triangle winds the same way:
+        // counter-clockwise in world XY viewed from above (+Z toward the
+        // viewer), i.e. cross2D(v1-v0, v2-v0) > 0. Varied heights + several
+        // landblock coords exercise both FSplitNESW split directions across
+        // the 64 cells. A future emission-order change that flips any
+        // triangle would silently punch terrain holes under culling.
+        var block = BuildFlatLandBlock();
+        for (int i = 0; i < 81; i++)
+            block.Height[i] = (byte)((i * 37) % 64);   // varied, deterministic slopes
+
+        foreach (var (lbx, lby) in new[] { (0u, 0u), (0xA9u, 0xB4u), (3u, 7u) })
+        {
+            var cache = new Dictionary<uint, SurfaceInfo>();
+            var mesh = LandblockMesh.Build(block, lbx, lby, IdentityHeightTable, MakeContext(), cache);
+
+            for (int t = 0; t < mesh.Indices.Length; t += 3)
+            {
+                var p0 = mesh.Vertices[mesh.Indices[t + 0]].Position;
+                var p1 = mesh.Vertices[mesh.Indices[t + 1]].Position;
+                var p2 = mesh.Vertices[mesh.Indices[t + 2]].Position;
+                float crossZ = (p1.X - p0.X) * (p2.Y - p0.Y) - (p1.Y - p0.Y) * (p2.X - p0.X);
+                Assert.True(crossZ > 0f,
+                    $"lb=({lbx},{lby}) triangle {t / 3} winds CW in world XY (crossZ={crossZ}) — " +
+                    "backface culling in TerrainModernRenderer would cull its TOP side");
+            }
+        }
+    }
+
     [Fact]
     public void Build_HeightmapPackedAsXMajor_NotYMajor()
     {