phase(N.5) Task 7: dispatcher SSBO + indirect buffer infrastructure

Adds DrawElementsIndirectCommand struct (20-byte layout for
glMultiDrawElementsIndirect). Replaces _instanceVbo field on
WbDrawDispatcher with three buffers: _instanceSsbo (mat4[]),
_batchSsbo (BatchData[]), _indirectBuffer (DEIC[]). Adds BindlessSupport
constructor parameter — non-null required since the dispatcher is only
constructed when WB foundation is on (which implies bindless is present
per Task 6 capability detection).

Existing Draw() method substitutes _instanceVbo -> _instanceSsbo for
compile. Behavior is temporarily wrong (SSBO bound as ArrayBuffer for
per-vertex attribs); Tasks 9-10 fully rewrite the draw loop and the
per-frame uploads to use BindBufferBase + glMultiDrawElementsIndirect.

GameWindow construction site updated to add _bindlessSupport guard and
pass it as the new last argument to the constructor. Dispatcher is only
constructed when bindless is guaranteed present.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -1524,10 +1524,11 @@ public sealed class GameWindow : IDisposable
         _staticMesh = new InstancedMeshRenderer(_gl, _meshShader, _textureCache, _wbMeshAdapter);
 
         if (AcDream.App.Rendering.Wb.WbFoundationFlag.IsEnabled
-            && _wbMeshAdapter is not null && _wbEntitySpawnAdapter is not null)
+            && _wbMeshAdapter is not null && _wbEntitySpawnAdapter is not null
+            && _bindlessSupport is not null)
         {
             _wbDrawDispatcher = new AcDream.App.Rendering.Wb.WbDrawDispatcher(
-                _gl, _meshShader, _textureCache, _wbMeshAdapter, _wbEntitySpawnAdapter);
+                _gl, _meshShader, _textureCache, _wbMeshAdapter, _wbEntitySpawnAdapter, _bindlessSupport);
         }
 
         // Phase G.1 sky renderer — its own shader (sky.vert / sky.frag)

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

@@ -0,0 +1,17 @@
+using System.Runtime.InteropServices;
+
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Layout matches what <c>glMultiDrawElementsIndirect</c> expects.
+/// Total size 20 bytes; arrays are typically uploaded with stride = sizeof(this).
+/// </summary>
+[StructLayout(LayoutKind.Sequential, Pack = 4)]
+public struct DrawElementsIndirectCommand
+{
+    public uint Count;          // index count for this draw
+    public uint InstanceCount;  // number of instances
+    public uint FirstIndex;     // offset into IBO, in indices
+    public int  BaseVertex;     // vertex offset into VBO
+    public uint BaseInstance;   // first instance ID (offsets per-instance attribs / SSBO read)
+}

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

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using System.Runtime.InteropServices;
 using AcDream.Core.Meshing;
 using AcDream.Core.Terrain;
 using AcDream.Core.World;
@@ -61,7 +62,32 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
     private readonly WbMeshAdapter _meshAdapter;
     private readonly EntitySpawnAdapter _entitySpawnAdapter;
 
-    private readonly uint _instanceVbo;
+    private readonly BindlessSupport _bindless;
+
+    // SSBO buffer ids
+    private uint _instanceSsbo;
+    private uint _batchSsbo;
+    private uint _indirectBuffer;
+
+    // Per-frame scratch arrays — Tasks 9-10 fully wire these.
+    private float[] _instanceData = new float[256 * 16];      // mat4 floats per instance
+    private BatchData[] _batchData = new BatchData[256];
+    private DrawElementsIndirectCommand[] _indirectCommands = new DrawElementsIndirectCommand[256];
+
+#pragma warning disable CS0169 // Tasks 9-10 wire these counters
+    private int _opaqueDrawCount;
+    private int _transparentDrawCount;
+    private int _transparentByteOffset;
+#pragma warning restore CS0169
+
+    [StructLayout(LayoutKind.Sequential, Pack = 4)]
+    private struct BatchData
+    {
+        public ulong TextureHandle;   // bindless handle (uvec2 in GLSL)
+        public uint  TextureLayer;
+        public uint  Flags;
+    }
+
     private readonly HashSet<uint> _patchedVaos = new();
 
     // Per-frame scratch — reused across frames to avoid per-frame allocation.
@@ -89,7 +115,8 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         Shader shader,
         TextureCache textures,
         WbMeshAdapter meshAdapter,
-        EntitySpawnAdapter entitySpawnAdapter)
+        EntitySpawnAdapter entitySpawnAdapter,
+        BindlessSupport bindless)
     {
         ArgumentNullException.ThrowIfNull(gl);
         ArgumentNullException.ThrowIfNull(shader);
@@ -103,7 +130,10 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         _meshAdapter = meshAdapter;
         _entitySpawnAdapter = entitySpawnAdapter;
 
-        _instanceVbo = _gl.GenBuffer();
+        _bindless = bindless ?? throw new ArgumentNullException(nameof(bindless));
+        _instanceSsbo   = _gl.GenBuffer();
+        _batchSsbo      = _gl.GenBuffer();
+        _indirectBuffer = _gl.GenBuffer();
     }
 
     public static Matrix4x4 ComposePartWorldMatrix(
@@ -291,7 +321,10 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         _opaqueDraws.Sort(static (a, b) => a.SortDistance.CompareTo(b.SortDistance));
 
         // ── Phase 3: one upload of all matrices ─────────────────────────────
-        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
+        // NOTE: _instanceSsbo is temporarily bound as ArrayBuffer for compile
+        // compatibility. Tasks 9-10 rewrite this to BindBufferBase(SSBO) +
+        // glMultiDrawElementsIndirect.
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceSsbo);
         fixed (float* p = _instanceBuffer)
             _gl.BufferData(BufferTargetARB.ArrayBuffer,
                 (nuint)(totalInstances * 16 * sizeof(float)), p, BufferUsageARB.DynamicDraw);
@@ -472,7 +505,9 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         if (!_patchedVaos.Add(vao)) return;
 
         _gl.BindVertexArray(vao);
-        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceVbo);
+        // NOTE: temporarily binding _instanceSsbo as ArrayBuffer for compile
+        // compatibility. Tasks 9-10 replace with BindBufferBase(SSBO).
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _instanceSsbo);
         for (uint row = 0; row < 4; row++)
         {
             uint loc = 3 + row;
@@ -494,7 +529,9 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
     {
         if (_disposed) return;
         _disposed = true;
-        _gl.DeleteBuffer(_instanceVbo);
+        _gl.DeleteBuffer(_instanceSsbo);
+        _gl.DeleteBuffer(_batchSsbo);
+        _gl.DeleteBuffer(_indirectBuffer);
     }
 
     private readonly record struct GroupKey(