chore(phys): A6.P3 #98 triage — revert neg-poly + bldg-check experiments

Triage step from the plan at C:\Users\erikn\.claude\plans\
i-did-some-work-sharded-acorn.md. Four sessions on issue #98 left the
worktree dirty with ~1352 LOC of mixed work. This commit splits the
work into "keep" (defensible + diagnostic) and "drop" (failed
experiments), then commits the keep set with the drops removed.

Plan asked for three commits (diag / fix / revert); consolidated to one
because the diagnostic emits in TransitionTypes.cs are tightly
interleaved with the multi-sphere CellTransit calls and the CellId
switch. Hunk-level splitting in those files for marginal bisect
granularity didn't justify the misclick risk.

Reverted entirely (failed experiments per slice 7 handoff):
- src/AcDream.Core/Physics/PhysicsDataCache.cs — neg-poly storage
  fields (Stippling, PosSurface, NegSurface, HasNegativeSide,
  IsNegativeSide, NegativeSide).
- src/AcDream.Core/Physics/ShadowObjectRegistry.cs — isBuilding flag
  propagation through Register / ShadowEntry.
- tests/AcDream.Core.Tests/Physics/BSPQueryTests.cs — 165 lines of
  PolygonWithNegativeSide_* tests.
- tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs —
  isBuilding propagation tests.
- src/AcDream.Core/World/WorldEntity.cs — IsLandblockBuilding field
  (no consumer once ShadowObjectRegistry.isBuilding is gone).
- src/AcDream.Core/World/LandblockLoader.cs — IsLandblockBuilding=true
  setter on building entities (kept BuildBuildingTerrainCells).
- src/AcDream.App/Rendering/GameWindow.cs — isBuilding: arg passed to
  ShadowObjects.Register.
- src/AcDream.Core/Physics/BSPQuery.cs — TryAdjustWalkableSide /
  IsWalkableAt helpers, their callers, the Path 5 / Path 6 neg-poly
  branch split, the BldgCheck-tied clearCell conditional, and the
  neg-poly ResolveCellPolygons writes.
- src/AcDream.Core/Physics/PhysicsDiagnostics.cs — neg-poly fields
  in the poly-dump format.
- src/AcDream.Core/Physics/TransitionTypes.cs — SpherePath.BldgCheck +
  SpherePath.HitsInteriorCell fields and every consumer, the
  savedBldgCheck try/finally around FindCollisions, and the neg-poly
  format additions to the dump-on-error helper.
- src/AcDream.Core/Physics/CellTransit.cs — FindCellSet overloads
  with hitsInteriorCell out-param and the BuildCellSetAndPickContaining
  out-param threading.

Kept (defensible correctness fixes + diagnostic infrastructure):
- src/AcDream.App/Rendering/GameWindow.cs — render-vs-physics cell
  origin split: the 0.02m render lift no longer leaks into physics
  BSP caching. lb.BuildingTerrainCells threaded into LandblockMesh.Build.
- src/AcDream.Core/World/LoadedLandblock.cs — BuildingTerrainCells
  record field.
- src/AcDream.Core/World/LandblockLoader.cs — BuildBuildingTerrainCells
  (cy*8+cx from LandBlockInfo.Buildings).
- src/AcDream.Core/Terrain/LandblockMesh.cs — hiddenTerrainCells
  param that collapses owned-cell triangles to a zero-area degenerate.
- src/AcDream.App/Streaming/{GpuWorldState,LandblockStreamer}.cs —
  mechanical BuildingTerrainCells threading through LoadedLandblock
  reconstructions.
- src/AcDream.Core/Physics/CellTransit.cs — multi-sphere
  FindTransitCellsSphere variant + multi-sphere AddAllOutsideCells +
  FindCellSet(IReadOnlyList<Sphere>, …) overload + the
  BSPQuery.SphereIntersectsCellBsp call for loaded neighbours. Matches
  retail CObjCell::find_cell_list / CEnvCell::find_transit_cells.
- src/AcDream.Core/Physics/TransitionTypes.cs — multi-sphere FindCellSet
  call site, retail-faithful CellId switch after CheckOtherCells, the
  outdoor-landcell terrain-walkable fallback in CheckOtherCells, and
  the full diagnostic suite ([step-walk], [walkable-nearest],
  [issue98-walkable-detail], [cell-set-summary], LastBspHitPoly
  emits).
- src/AcDream.Core/Physics/PhysicsDiagnostics.cs — ProbeStepWalkEnabled
  gate (ACDREAM_PROBE_STEP_WALK=1) + LogStepWalk helper + FormatVector
  / FormatPlane utilities. All emit-gated.
- src/AcDream.Core/Physics/BSPQuery.cs — diagnostic emits to
  LastBspHitPoly at four sites in SphereIntersectsPolyInternal /
  the placement adjustment path.
- Test files for the kept work: CellTransitFindCellSetTests,
  CellTransitFindTransitCellsSphereTests, PhysicsDiagnosticsTests,
  TransitionCheckOtherCellsTests, LandblockMeshTests,
  LandblockLoaderTests.

Verification:
- dotnet build: green, 0 errors, 3 pre-existing warnings.
- dotnet test: 1156 passed + 8 failed (baseline was 1148 + 8 pre-
  existing; the +8 passing are the new tests for the kept defensible
  work). Same 8 pre-existing failures, no new regressions.

Backup of pre-triage worktree state in stash@{0}.

A6.P3 #98 is still open; this is the apparatus-prep step, not a fix.
Next: cell-dump probe (Step 2 of the plan).

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

@@ -54,6 +54,35 @@ public class LandblockMeshTests
         Assert.Equal(128 * 3, mesh.Indices.Length);
     }
 
+    [Fact]
+    public void Build_HiddenTerrainCell_PreservesCountsAndDegeneratesOnlyThatCell()
+    {
+        var block = BuildFlatLandBlock();
+        var cache = new Dictionary<uint, SurfaceInfo>();
+        int hiddenCell = (3 * LandblockMesh.CellsPerSide) + 5;
+
+        var mesh = LandblockMesh.Build(
+            block,
+            0,
+            0,
+            IdentityHeightTable,
+            MakeContext(),
+            cache,
+            new HashSet<int> { hiddenCell });
+
+        Assert.Equal(LandblockMesh.VerticesPerLandblock, mesh.Vertices.Length);
+        Assert.Equal(LandblockMesh.VerticesPerLandblock, mesh.Indices.Length);
+
+        int hiddenBase = hiddenCell * LandblockMesh.VerticesPerCell;
+        for (int i = 0; i < LandblockMesh.VerticesPerCell; i++)
+            Assert.Equal((uint)hiddenBase, mesh.Indices[hiddenBase + i]);
+
+        int visibleCell = hiddenCell + 1;
+        int visibleBase = visibleCell * LandblockMesh.VerticesPerCell;
+        for (int i = 0; i < LandblockMesh.VerticesPerCell; i++)
+            Assert.Equal((uint)(visibleBase + i), mesh.Indices[visibleBase + i]);
+    }
+
     [Fact]
     public void Build_Vertices_CoverExactly192x192WorldUnits()
     {