feat(render): Phase A8 Wave 3 — wire EnvCellRenderer into landblock streaming

Six surgical edits to GameWindow.cs (+1 MeshManager accessor on WbMeshAdapter):

1. Field declarations (line 166-167): _envCellRenderer + _envCellFrustum.
2. Ctor init (line 1775-1778): construct WbFrustum + EnvCellRenderer,
   Initialize with the existing _meshShader (loaded from mesh_modern.vert/frag).
3. BuildInteriorEntitiesForStreaming (line 5444): _envCellRenderer.RegisterCell(...)
   replaces the cell-as-WorldEntity creation block. staticObjects is empty —
   cell stabs continue as WorldEntity records via the dispatcher's IndoorPass.
4. ApplyLoadedTerrainLocked (line 5885): _envCellRenderer.FinalizeLandblock(...)
   immediately after _buildingRegistries[lb.LandblockId] = ... — atomically
   commits the landblock's per-cell instance store.
5. RemoveLandblock callbacks (lines 1861 + 8955): mirror the existing
   _buildingRegistries.Remove(id) sites so EnvCellRenderer's storage clears
   in lockstep.
6. Dispose (line 10595): _envCellRenderer?.Dispose() after _wbDrawDispatcher.

Plan revision (vs original plan.md Task 6): we keep the static-object stab
WorldEntity hydration (lines 5440-5489) instead of deleting it — stabs need
WorldEntity records for interaction (clicking) and physics. EnvCellRenderer
receives empty staticObjects so it only renders cell geometry; stab rendering
continues unchanged through the dispatcher.

Build green. 23/23 EnvCellRenderer + WbFrustum + EnvCellSceneryInstance
tests pass. App.Tests baseline holds (82/82). Pre-existing Core.Tests
static-leak flakiness (8-19 failures, documented baseline) unrelated.

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

src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs

@@ -120,6 +120,13 @@ public sealed class WbMeshAdapter : IDisposable, IWbMeshAdapter
     /// </summary>
     public AcSurfaceMetadataTable MetadataTable => _metadataTable;
 
+    /// <summary>
+    /// Phase A8 (2026-05-28): exposes the underlying <see cref="ObjectMeshManager"/>
+    /// so <c>EnvCellRenderer</c> can share the same global mesh buffer (VAO/VBO/IBO).
+    /// Returns null when the adapter is uninitialized.
+    /// </summary>
+    public ObjectMeshManager? MeshManager => _meshManager;
+
     /// <summary>
     /// Returns the WB render data for <paramref name="id"/>, or null if not
     /// yet uploaded or if this adapter is uninitialized. Increments WB's