diff --git a/tests/AcDream.Core.Tests/Physics/Issue108CellarAscentViewerReplayTests.cs b/tests/AcDream.Core.Tests/Physics/Issue108CellarAscentViewerReplayTests.cs
new file mode 100644
index 00000000..abd691a8
--- /dev/null
+++ b/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;
+
+///
+/// #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.
+///
+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 envCells)
+ BuildEngine(DatCollection dats)
+ {
+ var cache = new PhysicsDataCache();
+ var engine = new PhysicsEngine { DataCache = cache };
+ var envCells = new Dictionary();
+
+ // 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(Lb | 0xFFFEu);
+ Assert.NotNull(lbInfo);
+ foreach (var building in lbInfo!.Buildings)
+ {
+ if (building.Portals.Count == 0) continue;
+ var portals = new List(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(), Array.Empty(), 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 ──────────────────────────────────────────────────────
+
+ /// Stair-line feet Z for a path y (fitted from the capture bands).
+ 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? 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();
+ 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 ──────────────────────────────────────────────
+
+ ///
+ /// 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.
+ ///
+ [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) ---"));
+ }
+
+ /// Boom-distance + damping-lag sweep: how wide is the window across poses?
+ [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(
+ $"dist={dist:F2} lag={lag:F2}: grassWindow={grass}/{steps.Count} sweepOkFalse={okFalse} fallbackBranch={fb}"));
+ }
+ }
+
+ ///
+ /// THE PIN: while the eye is below terrain grade on the cellar ascent, the
+ /// viewer must resolve INTERIOR — an outdoor/null viewer cell roots the
+ /// frame at the landscape and sweeps grass across the exit door (#108).
+ /// Retail's viewer rides the stairwell cells here (the cellar camera works
+ /// in retail); below grade inside the building footprint there is no
+ /// legitimate outdoor viewer.
+ ///
+ [Fact]
+ public void CellarAscent_ViewerStaysInterior_WhileEyeBelowGrade()
+ {
+ var steps = RunAscent(BoomDistance, pathLagMeters: 0f);
+ if (steps is null) return;
+
+ var failures = steps.FindAll(s => s.EyeBelowGrade && s.ViewerOutdoorOrNull);
+ if (failures.Count > 0)
+ {
+ _out.WriteLine($"--- {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)");
+ }
+}