feat(app): Phase A.1 — TerrainRenderer.RemoveLandblock for streaming unloads

TerrainRenderer's internal landblock collection is now a Dictionary
keyed by landblock id so the streaming system can release GPU
resources per-landblock as the visible window moves. AddLandblock
takes the id as its first parameter; if the same id is added twice,
the old buffers are freed before the new ones land (defensive but
cheap). RemoveLandblock is a no-op for unknown ids and deletes
VBO/EBO/VAO for known ones.

Single existing caller in GameWindow.cs updated to pass the id.

Build green. No unit tests — direct-to-GL methods need a live context.
Tasks 5-7 will validate end-to-end.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -245,7 +245,7 @@ public sealed class GameWindow : IDisposable
                 ((int)lbY - centerY) * 192f,
                 0f);
 
-            _terrain.AddLandblock(meshData, origin);
+            _terrain.AddLandblock(lb.LandblockId, meshData, origin);
         }
         Console.WriteLine($"terrain: {surfaceCache.Count} unique palette codes across {worldView.Landblocks.Count} landblocks");
 

src/AcDream.App/Rendering/TerrainRenderer.cs

@@ -22,7 +22,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
     private readonly GL _gl;
     private readonly Shader _shader;
     private readonly TerrainAtlas _atlas;
-    private readonly List<LandblockGpu> _landblocks = new();
+    private readonly Dictionary<uint, LandblockGpu> _landblocks = new();
 
     public TerrainRenderer(GL gl, Shader shader, TerrainAtlas atlas)
     {
@@ -31,10 +31,19 @@ public sealed unsafe class TerrainRenderer : IDisposable
         _atlas = atlas;
     }
 
-    public void AddLandblock(LandblockMeshData meshData, Vector3 worldOrigin)
+    public void AddLandblock(uint landblockId, LandblockMeshData meshData, Vector3 worldOrigin)
     {
+        if (_landblocks.TryGetValue(landblockId, out var existing))
+        {
+            _gl.DeleteBuffer(existing.Vbo);
+            _gl.DeleteBuffer(existing.Ebo);
+            _gl.DeleteVertexArray(existing.Vao);
+            _landblocks.Remove(landblockId);
+        }
+
         var gpu = new LandblockGpu
         {
+            LandblockId = landblockId,
             Vao = _gl.GenVertexArray(),
             WorldOrigin = worldOrigin,
             IndexCount = meshData.Indices.Length,
@@ -76,7 +85,23 @@ public sealed unsafe class TerrainRenderer : IDisposable
         _gl.VertexAttribIPointer(5, 4, VertexAttribIType.UnsignedByte, stride, (void*)(dataOffset + 12));
 
         _gl.BindVertexArray(0);
-        _landblocks.Add(gpu);
+        _landblocks[landblockId] = gpu;
+    }
+
+    /// <summary>
+    /// Release GPU buffers for a previously-added landblock. No-op if the
+    /// landblock wasn't added. Called by the streaming system when a
+    /// landblock falls outside the visible window.
+    /// </summary>
+    public void RemoveLandblock(uint landblockId)
+    {
+        if (!_landblocks.TryGetValue(landblockId, out var gpu))
+            return;
+
+        _gl.DeleteBuffer(gpu.Vbo);
+        _gl.DeleteBuffer(gpu.Ebo);
+        _gl.DeleteVertexArray(gpu.Vao);
+        _landblocks.Remove(landblockId);
     }
 
     public void Draw(ICamera camera)
@@ -96,7 +121,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
         int alphaLoc = _gl.GetUniformLocation(_shader.Program, "uAlpha");
         if (alphaLoc >= 0) _gl.Uniform1(alphaLoc, 1);
 
-        foreach (var lb in _landblocks)
+        foreach (var lb in _landblocks.Values)
         {
             var model = Matrix4x4.CreateTranslation(lb.WorldOrigin);
             _shader.SetMatrix4("uModel", model);
@@ -108,7 +133,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
 
     public void Dispose()
     {
-        foreach (var lb in _landblocks)
+        foreach (var lb in _landblocks.Values)
         {
             _gl.DeleteBuffer(lb.Vbo);
             _gl.DeleteBuffer(lb.Ebo);
@@ -119,6 +144,7 @@ public sealed unsafe class TerrainRenderer : IDisposable
 
     private sealed class LandblockGpu
     {
+        public uint LandblockId;
         public uint Vao;
         public uint Vbo;
         public uint Ebo;