phase(N.4) Task 12: wire LandblockSpawnAdapter into GpuWorldState

GpuWorldState's constructor accepts an optional LandblockSpawnAdapter.
AddLandblock calls OnLandblockLoaded with the post-merge loaded record;
RemoveLandblock calls OnLandblockUnloaded with the landblock id at the
top of the method (before state mutation).

Both calls are gated behind WbFoundationFlag.IsEnabled — no behavioral
change with flag off (existing tests pass without modification).

GameWindow constructs the adapter under the flag and threads it into
GpuWorldState. With flag on, atlas-tier scenery now drives WB ref
counts; per-instance entities (ServerGuid != 0) are filtered out by
the adapter and don't reach WB.

Foundation for Task 13 (memory budget verification under stress).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/AcDream.App/Streaming/GpuWorldState.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Numerics;
+using AcDream.App.Rendering.Wb;
 using AcDream.Core.World;
 
 namespace AcDream.App.Streaming;
@@ -38,6 +39,13 @@ namespace AcDream.App.Streaming;
 /// </summary>
 public sealed class GpuWorldState
 {
+    private readonly LandblockSpawnAdapter? _wbSpawnAdapter;
+
+    public GpuWorldState(LandblockSpawnAdapter? wbSpawnAdapter = null)
+    {
+        _wbSpawnAdapter = wbSpawnAdapter;
+    }
+
     private readonly Dictionary<uint, LoadedLandblock> _loaded = new();
     private readonly Dictionary<uint, (Vector3 Min, Vector3 Max)> _aabbs = new();
 
@@ -132,6 +140,8 @@ public sealed class GpuWorldState
         }
 
         _loaded[landblock.LandblockId] = landblock;
+        if (WbFoundationFlag.IsEnabled && _wbSpawnAdapter is not null)
+            _wbSpawnAdapter.OnLandblockLoaded(_loaded[landblock.LandblockId]);
         RebuildFlatView();
     }
 
@@ -181,6 +191,9 @@ public sealed class GpuWorldState
 
     public void RemoveLandblock(uint landblockId)
     {
+        if (WbFoundationFlag.IsEnabled && _wbSpawnAdapter is not null)
+            _wbSpawnAdapter.OnLandblockUnloaded(landblockId);
+
         // Rescue persistent entities before removal. These get appended
         // to the _persistentRescued list; the caller is responsible for
         // re-injecting them (via AppendLiveEntity) into whatever landblock