fix(render): Phase A8 — stamp BuildingId on already-loaded cells too

First visual-gate launch showed 8,737 [vis] lines (player at Holtburg
cottage cell 0xA9B40143, inside=True really=True) but ZERO [buildings] /
[envcells] / [stencil] / [draworder] probe emissions. Root cause: same as
the original RR7.1 saga — BuildingLoader.Build was passed only the
per-frame drainedCells dict, missing cells loaded on PRIOR frames. Those
cells stayed with BuildingId=null, the strict cameraInsideBuilding gate
returned false, the indoor branch never fired.

Fix: in ApplyLoadedTerrainLocked, merge drainedCells with the cells
already registered in _cellVisibility for the same landblock prefix
before passing to BuildingLoader. The richer dict ensures the stamping
loop in BuildingLoader.Build covers EVERY cell in this landblock.

Added IReadOnlyList<LoadedCell> GetCellsForLandblock(uint lbId) on
CellVisibility — minimal API expose; existing _cellsByLandblock dict
was already the right shape (lbId = upper 16 bits).

Build green. Tests unchanged.

Next: relaunch the client. With the fix, [buildings] probe should fire
with camBldgs=[0x1,...] when the player is inside a Holtburg cottage,
[envcells] should report cells>=1 tris>=1 per indoor frame, and the
indoor branch should be exercising the WB-faithful Steps 1-5 pipeline.

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

src/AcDream.App/Rendering/CellVisibility.cs

@@ -208,6 +208,23 @@ public sealed class CellVisibility
         _cellLookup[cell.CellId] = cell;
     }
 
+    /// <summary>
+    /// Phase A8 (2026-05-28): enumerates the loaded cells that belong to a
+    /// landblock prefix. Used by <c>GameWindow.ApplyLoadedTerrainLocked</c> when
+    /// building the per-landblock <c>BuildingRegistry</c> — the per-frame
+    /// <c>drainedCells</c> dict misses cells loaded on prior frames, so the
+    /// stamping loop in <see cref="Wb.BuildingLoader.Build"/> needs access to
+    /// every cell currently in the landblock to ensure <c>BuildingId</c> is set.
+    /// </summary>
+    /// <param name="lbId">Upper 16 bits of the landblock key (e.g. <c>0xA9B4</c>
+    /// for landblock <c>0xA9B40000</c>). NOT the full 32-bit landblock id.</param>
+    public IReadOnlyList<LoadedCell> GetCellsForLandblock(uint lbId)
+    {
+        return _cellsByLandblock.TryGetValue(lbId, out var list)
+            ? list
+            : System.Array.Empty<LoadedCell>();
+    }
+
     /// <summary>
     /// Removes all cells belonging to <paramref name="lbId"/> (upper 16 bits of
     /// the landblock key, e.g. 0xA9B4 for landblock 0xA9B40000). Called when a