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/Physics/CellTransitFindCellSetTests.cs

@@ -134,4 +134,43 @@ public class CellTransitFindCellSetTests
         Assert.Equal(0xA9B40001u, containing);
         Assert.True(cellSet.Count >= 2, $"Expected ≥2 cells in set (primary + east neighbour), got {cellSet.Count}");
     }
+
+    [Fact]
+    public void IndoorSeed_ExitPortalTouchedOnlyBySecondSphere_AddsOutdoorLandcell()
+    {
+        // Retail CObjCell::find_cell_list passes every SPHEREPATH sphere into
+        // CEnvCell::find_transit_cells. When any sphere straddles an outdoor
+        // exit portal, CLandCell::add_all_outside_cells runs for the whole
+        // sphere array.
+        uint lbPrefix = 0xA9B40000u;
+        float lbX = ((lbPrefix >> 24) & 0xFFu) * 192f;
+        float lbY = ((lbPrefix >> 16) & 0xFFu) * 192f;
+
+        var cellTransform = Matrix4x4.CreateTranslation(new Vector3(lbX, lbY, 0f));
+        var exitCell = MakeCellWithPortalAtRightWall(
+            cellTransform,
+            otherCellId: 0xFFFF,
+            flags: 0);
+
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40100u, exitCell);
+
+        var spheres = new[]
+        {
+            // Foot sphere is not near the exit portal plane at local x=2.5.
+            new Sphere { Origin = new Vector3(lbX + 0.0f, lbY + 12.0f, 2.5f), Radius = 0.5f },
+            // Head sphere reaches the exit portal plane and should trigger
+            // outdoor landcell expansion.
+            new Sphere { Origin = new Vector3(lbX + 2.0f, lbY + 12.0f, 3.2f), Radius = 0.5f },
+        };
+
+        uint containing = CellTransit.FindCellSet(
+            cache, spheres, spheres.Length,
+            currentCellId: 0xA9B40100u,
+            out var cellSet);
+
+        Assert.Equal(0xA9B40100u, containing);
+        Assert.Contains(0xA9B40100u, cellSet);
+        Assert.Contains(0xA9B40001u, cellSet);
+    }
 }