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>

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

@@ -28,4 +28,17 @@ public class ResolveCellIdTests
 
         Assert.Equal(0xA9B40001u, result);
     }
+
+    [Fact]
+    public void ResolveCellId_NoDataCache_ReturnsFallback()
+    {
+        // Build a PhysicsEngine without setting DataCache.
+        var engine = new PhysicsEngine { DataCache = null };
+        uint result = engine.ResolveCellId(
+            new Vector3(100, 100, 0),
+            sphereRadius: 0.5f,
+            fallbackCellId: 0xA9B40100u);  // indoor seed
+        // Indoor branch falls back when DataCache is null.
+        Assert.Equal(0xA9B40100u, result);
+    }
 }