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/PhysicsEngine.cs

@@ -258,7 +258,8 @@ public sealed class PhysicsEngine
 
         if (fallbackLow >= 0x0100u)
         {
-            // Indoor seed: use portal-graph traversal.
+            // Indoor branch needs DataCache to look up cells; outdoor uses
+            // _landblocks (no DataCache dependency).
             if (DataCache is null) return fallbackCellId;
             return CellTransit.FindCellList(DataCache, worldPos, sphereRadius, fallbackCellId);
         }
@@ -274,7 +275,29 @@ public sealed class PhysicsEngine
             if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
             {
                 uint lowCellId = lb.Terrain.ComputeOutdoorCellId(localX, localY);
-                return (kvp.Key & 0xFFFF0000u) | lowCellId;
+                uint outdoorCellId = (kvp.Key & 0xFFFF0000u) | lowCellId;
+
+                // Outdoor→indoor entry: if this landcell has a cached building,
+                // check whether the sphere has crossed into one of its interior
+                // EnvCells via the building's portals.
+                if (DataCache is not null)
+                {
+                    var building = DataCache.GetBuilding(outdoorCellId);
+                    if (building is not null)
+                    {
+                        var candidates = new System.Collections.Generic.HashSet<uint>();
+                        CellTransit.CheckBuildingTransit(
+                            DataCache, building, worldPos, sphereRadius, candidates);
+                        if (candidates.Count > 0)
+                        {
+                            // First candidate wins — building portal containment is
+                            // mutually exclusive in retail (one interior cell per portal).
+                            foreach (var c in candidates) return c;
+                        }
+                    }
+                }
+
+                return outdoorCellId;
             }
         }