feat(render): Phase A8 RR4 — wire BuildingRegistry into landblock load

LoadedCell.BuildingId (init + internal setter) — set exactly once at
    landblock load time by BuildingLoader; null when the cell isn't
    part of any building (outdoor surface cells; dungeon cells not
    enumerated in LandBlockInfo.Buildings).

  GameWindow landblock-load path: builds BuildingRegistry from
    LandBlockInfo.Buildings; stamps each cell's BuildingId; stores the
    registry on _buildingRegistries[landblockId] (GameWindow-level dict)
    for render-frame lookups. Note: LoadedLandblock is AcDream.Core.World
    (a sealed record) — adding an App-type field there would violate
    Code Structure Rule #2, so the registry is stored in a new
    GameWindow-level dictionary instead. Cleanup wired in both
    removeTerrain lambdas (OnLoad + OnResize paths).

  drainedCells dict: the existing _pendingCells drain loop is extended
    to also build a local CellId→LoadedCell dict; BuildingLoader.Build
    uses this dict for the stamping pass so no second iteration is needed.

  New BuildingLoaderTest verifies the stamping path. 5 BuildingLoader
  tests total (4 from RR3 + 1 new).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/CellVisibility.cs

@@ -70,6 +70,19 @@ public sealed class LoadedCell
     /// </para>
     /// </summary>
     public List<Vector3[]> PortalPolygons = new();
+
+    /// <summary>
+    /// Phase A8 (2026-05-26): the building this cell belongs to, if any.
+    /// Set exactly once by <see cref="Wb.BuildingLoader"/> immediately after
+    /// LandblockLoader produces the cells. Null when the cell isn't part of
+    /// any building (outdoor surface cells; dungeon cells not enumerated in
+    /// LandBlockInfo.Buildings).
+    ///
+    /// <para>Used by the render frame to derive the camera-buildings set
+    /// via <see cref="Wb.BuildingRegistry.GetBuildingsContainingCell"/>
+    /// and route IndoorPass cell scoping.</para>
+    /// </summary>
+    public uint? BuildingId { get; internal set; }
 }
 
 /// <summary>