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/BuildingPhysics.cs

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Indoor walking Phase 2 (2026-05-19). Cached building portal data
+/// for outdoor→indoor cell entry. One per outdoor landcell that contains
+/// a building stab. Mirrors retail's <c>BuildingObj.Portals</c> array
+/// (per the pseudocode doc §"LandCell.find_transit_cells").
+/// </summary>
+public sealed class BuildingPhysics
+{
+    public required Matrix4x4 WorldTransform { get; init; }
+    public required Matrix4x4 InverseWorldTransform { get; init; }
+    public required IReadOnlyList<BldPortalInfo> Portals { get; init; }
+}
+
+/// <summary>
+/// One building portal: the connection from a SortCell's BuildingObj to
+/// an interior EnvCell. ExactMatch is decoded from <see cref="Flags"/>
+/// bit 0 (<c>PortalFlags.ExactMatch = 0x0001</c>).
+/// </summary>
+public readonly struct BldPortalInfo
+{
+    public BldPortalInfo(uint otherCellId, ushort otherPortalId, ushort flags)
+    {
+        OtherCellId   = otherCellId;
+        OtherPortalId = otherPortalId;
+        Flags         = flags;
+    }
+
+    /// <summary>Full id of the interior EnvCell this portal connects to.</summary>
+    public uint   OtherCellId   { get; }
+    /// <summary>The portal id within the destination EnvCell.</summary>
+    public ushort OtherPortalId { get; }
+    public ushort Flags         { get; }
+
+    /// <summary>Bit 0 of Flags (<c>PortalFlags.ExactMatch = 0x0001</c>).</summary>
+    public bool ExactMatch => (Flags & 0x0001) != 0;
+}