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/GameWindow.cs

@@ -150,6 +150,14 @@ public sealed class GameWindow : IDisposable
     // to _cellVisibility on the render thread in ApplyLoadedTerrain.
     private readonly System.Collections.Concurrent.ConcurrentBag<LoadedCell> _pendingCells = new();
 
+    // Phase A8 (2026-05-26): per-landblock BuildingRegistry keyed by full landblock
+    // id (e.g. 0xA9B40000). Built from LandBlockInfo.Buildings at ApplyLoadedTerrain
+    // time; each entry's BuildingRegistry.GetBuildingsContainingCell drives render-frame
+    // indoor-cell scoping. Entries are removed in the removeTerrain callbacks.
+    // Only touched on the render thread — no lock required.
+    private readonly System.Collections.Generic.Dictionary<uint, AcDream.App.Rendering.Wb.BuildingRegistry>
+        _buildingRegistries = new();
+
     /// <summary>
     /// Phase 6.4: per-entity animation playback state for entities whose
     /// MotionTable resolved to a real cycle. The render loop ticks each
@@ -1833,6 +1841,7 @@ public sealed class GameWindow : IDisposable
                 _terrain?.RemoveLandblock(id);
                 _physicsEngine.RemoveLandblock(id);
                 _cellVisibility.RemoveLandblock((id >> 16) & 0xFFFFu);
+                _buildingRegistries.Remove(id);  // Phase A8
             });
         // A.5 T22.5: apply max-completions from resolved quality.
         _streamingController.MaxCompletionsPerFrame = _resolvedQuality.MaxCompletionsPerFrame;
@@ -5665,8 +5674,13 @@ public sealed class GameWindow : IDisposable
         _terrain.AddLandblockWithMesh(lb.LandblockId, meshData, origin);
 
         // Step 4: drain pending LoadedCells from the worker thread.
+        // Also collect into a local dict for the BuildingLoader stamping pass below.
+        var drainedCells = new System.Collections.Generic.Dictionary<uint, LoadedCell>();
         while (_pendingCells.TryTake(out var cell))
+        {
             _cellVisibility.AddCell(cell);
+            drainedCells[cell.CellId] = cell;
+        }
 
         // Compute the per-landblock AABB for frustum culling. XY from the
         // landblock's world origin + 192 footprint. Z from the terrain vertex
@@ -5826,6 +5840,20 @@ public sealed class GameWindow : IDisposable
 
             _physicsEngine.AddLandblock(lb.LandblockId, terrainSurface, cellSurfaces,
                 portalPlanes, origin.X, origin.Y);
+
+            // Phase A8 (2026-05-26): build per-landblock BuildingRegistry from
+            // LandBlockInfo.Buildings, stamping LoadedCell.BuildingId for each cell
+            // in a building's cell set. Uses the already-drained drainedCells dict
+            // (LoadedCells registered this frame) so stamping and registry build
+            // happen in the same render-thread pass — no extra dat reads required.
+            // Cells without a building stay at BuildingId == null (outdoor surface
+            // cells; dungeon cells not enumerated in LandBlockInfo.Buildings).
+            if (lbInfo is not null)
+            {
+                _buildingRegistries[lb.LandblockId] =
+                    AcDream.App.Rendering.Wb.BuildingLoader.Build(
+                        lbInfo, lb.LandblockId, drainedCells);
+            }
         }
 
         // N.5: WbMeshAdapter.Tick() handles GPU upload for all GfxObj meshes via
@@ -8894,6 +8922,7 @@ public sealed class GameWindow : IDisposable
                     _terrain?.RemoveLandblock(id);
                     _physicsEngine.RemoveLandblock(id);
                     _cellVisibility.RemoveLandblock((id >> 16) & 0xFFFFu);
+                    _buildingRegistries.Remove(id);  // Phase A8
                 });
             _streamingController.MaxCompletionsPerFrame = newResolved.MaxCompletionsPerFrame;