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>

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -20,6 +20,9 @@ public sealed class PhysicsDataCache
     private readonly ConcurrentDictionary<uint, SetupPhysics> _setup = new();
     private readonly ConcurrentDictionary<uint, CellPhysics> _cellStruct = new();
 
+    // ── Phase 2: building portal cache for outdoor→indoor entry ───────────
+    private readonly ConcurrentDictionary<uint, BuildingPhysics> _buildings = new();
+
     /// <summary>
     /// Extract and cache the physics BSP + polygon data from a GfxObj,
     /// PLUS always cache a visual AABB from the vertex data regardless of
@@ -304,6 +307,31 @@ public sealed class PhysicsDataCache
     /// </summary>
     public void RegisterCellStructForTest(uint envCellId, CellPhysics physics)
         => _cellStruct[envCellId] = physics;
+
+    /// <summary>
+    /// Indoor walking Phase 2 (2026-05-19). Cache the building portal list
+    /// for an outdoor landcell that contains a building stab. Used by
+    /// <see cref="CellTransit.CheckBuildingTransit"/>.
+    /// </summary>
+    public void CacheBuilding(uint landcellId, IReadOnlyList<BldPortalInfo> portals, Matrix4x4 worldTransform)
+    {
+        if (_buildings.ContainsKey(landcellId)) return;
+        Matrix4x4.Invert(worldTransform, out var inverse);
+        _buildings[landcellId] = new BuildingPhysics
+        {
+            WorldTransform = worldTransform,
+            InverseWorldTransform = inverse,
+            Portals = portals,
+        };
+    }
+
+    public BuildingPhysics? GetBuilding(uint landcellId)
+        => _buildings.TryGetValue(landcellId, out var b) ? b : null;
+
+    public IReadOnlyCollection<uint> BuildingIds => (IReadOnlyCollection<uint>)_buildings.Keys;
+
+    /// <summary>Test helper, mirrors <see cref="RegisterCellStructForTest"/>.</summary>
+    public void RegisterBuildingForTest(uint landcellId, BuildingPhysics b) => _buildings[landcellId] = b;
 }
 
 /// <summary>