refactor(terrain): chunk-based TerrainChunkRenderer matching ACME architecture

Replace the single-giant-buffer TerrainRenderer with TerrainChunkRenderer
that groups landblocks into 16×16 chunks, each with its own VAO/VBO/EBO.
Matches ACME's ChunkMetrics/ChunkRenderData/TerrainGPUResourceManager
pattern:

- Pre-allocated max-size VBO (~3.75MB) + EBO per chunk with DynamicDraw
- Incremental glBufferSubData uploads per landblock slot (no full rebuild)
- Fixed slot layout: slot = (localX * 16 + localY), vertBase = slot * 384
- EBO contains only indices for occupied slots → tight draw calls
- One DrawElements per chunk with chunk-level AABB frustum culling
- Empty chunks auto-dispose GPU resources

Streaming-friendly: add/remove a single landblock touches only its slot
in the chunk buffer, not the entire terrain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -16,7 +16,7 @@ public sealed class GameWindow : IDisposable
     private IWindow? _window;
     private GL? _gl;
     private IInputContext? _input;
-    private TerrainRenderer? _terrain;
+    private TerrainChunkRenderer? _terrain;
     private Shader? _shader;
     private CameraController? _cameraController;
     private IMouse? _capturedMouse;
@@ -339,7 +339,7 @@ public sealed class GameWindow : IDisposable
         // Build the terrain atlas once from the Region dat.
         var terrainAtlas = AcDream.App.Rendering.TerrainAtlas.Build(_gl, _dats);
 
-        _terrain = new TerrainRenderer(_gl, _shader, terrainAtlas);
+        _terrain = new TerrainChunkRenderer(_gl, _shader, terrainAtlas);
 
         int centerX = (int)((centerLandblockId >> 24) & 0xFFu);
         int centerY = (int)((centerLandblockId >> 16) & 0xFFu);