refactor(rendering): introduce InstancedMeshRenderer with GfxObj grouping

Groups all (entity, meshRef) pairs by GfxObjId before drawing so each
GfxObj's sub-meshes are processed as a contiguous batch.  Still uses
per-entity uniform uModel — visual output is identical to the old
StaticMeshRenderer — but the _groups dict is the structural prerequisite
for swapping to DrawElementsInstanced in the follow-up commit.

Key changes:
- New InstancedMeshRenderer.cs with CollectGroups() that fills
  _groups[gfxObjId] = List<InstanceEntry> each frame, reusing the
  inner List<> objects to avoid per-frame allocation.
- Same two-pass (opaque+clipmap first, translucent second) draw logic
  from StaticMeshRenderer, now iterating over groups rather than raw
  entity/meshRef pairs.
- GameWindow.cs: field and constructor swapped from StaticMeshRenderer
  to InstancedMeshRenderer — public API is identical.
- 431 tests green, 0 warnings.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -23,7 +23,7 @@ public sealed class GameWindow : IDisposable
     private DatCollection? _dats;
     private float _lastMouseX;
     private float _lastMouseY;
-    private StaticMeshRenderer? _staticMesh;
+    private InstancedMeshRenderer? _staticMesh;
     private Shader? _meshShader;
     private TextureCache? _textureCache;
 
@@ -370,7 +370,7 @@ public sealed class GameWindow : IDisposable
         _surfaceCache = new Dictionary<uint, AcDream.Core.Terrain.SurfaceInfo>();
 
         _textureCache = new TextureCache(_gl, _dats);
-        _staticMesh = new StaticMeshRenderer(_gl, _meshShader, _textureCache);
+        _staticMesh = new InstancedMeshRenderer(_gl, _meshShader, _textureCache);
 
         // Phase A.1: replace the one-shot 3×3 preload with a streaming controller.
         // Parse runtime radius from environment (default 2 → 5×5 window).