fix(O-T4): address spec-review findings — InstanceData + using cleanups

Four fixes from T4 spec review:

1. Extracted InstanceData.cs (14-line struct) verbatim to
   src/AcDream.App/Rendering/Wb/InstanceData.cs (per O-D1).

2. ObjectMeshManager.cs: replaced `using Chorizite.OpenGLSDLBackend.Lib;`
   with `using AcDream.Core.Rendering.Wb;` (TextureHelpers comes from
   our T2 Core extraction; InstanceData comes from new T4 cleanup).

3. EmbeddedResourceReader.GetEmbeddedResource promoted from `internal`
   to `public` per O-D9 intent (the type promotion only changed the
   class signature in T3; this finishes the spec).

4. OpenGLGraphicsDevice.cs: removed stale T3 interim comment at
   lines 142-145 — T4 resolved the ParticleBatcher construction
   via post-ctor assignment in WbMeshAdapter.cs:78.

Build green; tests green (1147 passing, 8 pre-existing failures
baseline maintained).

Spec: docs/superpowers/specs/2026-05-21-phase-o-dat-path-unification-design.md

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

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

@@ -16,7 +16,7 @@ namespace AcDream.App.Rendering.Wb {
     /// (lower-case, wb_ prefix to distinguish WB-origin shaders from acdream's own)
     /// </summary>
     public static class EmbeddedResourceReader {
-        internal static string GetEmbeddedResource(string filename) {
+        public static string GetEmbeddedResource(string filename) {
             // Convert "Shaders.Particle.vert" → "wb_particle.vert"
             // Strip leading "Shaders." then lowercase and prefix with wb_
             string leafName;

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

@@ -0,0 +1,15 @@
+using System.Numerics;
+using System.Runtime.InteropServices;
+
+namespace AcDream.App.Rendering.Wb {
+    [StructLayout(LayoutKind.Sequential, Pack = 16)]
+    public struct InstanceData {
+        public const uint INSTANCE_FLAG_DISQUALIFIED = 1u;
+
+        public Matrix4x4 Transform; // 64 bytes
+        public uint CellId;         // 4 bytes
+        public uint Flags;          // 4 bytes
+        private uint _pad1;         // 4 bytes
+        private uint _pad2;         // 4 bytes -> total 80 bytes
+    }
+}

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

@@ -16,7 +16,7 @@ using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
 using WorldBuilder.Shared.Services;
-using Chorizite.OpenGLSDLBackend.Lib;
+using AcDream.Core.Rendering.Wb;
 using PixelFormat = Silk.NET.OpenGL.PixelFormat;
 using BoundingBox = Chorizite.Core.Lib.BoundingBox;
 using BCnEncoder.Decoder;

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

@@ -139,10 +139,9 @@ namespace AcDream.App.Rendering.Wb {
 
             InitializeSharedDebugResources();
 
-            // T3 interim: ParticleBatcher (Chorizite.OpenGLSDLBackend.Lib.ParticleBatcher) is a T4 type
-            // (Particle batcher + emitter extraction is in T4). It expects the WB-original
-            // OpenGLGraphicsDevice type; we cannot pass `this` until T4 extracts it alongside us.
-            // The property is set to null! here; T4 will restore the real construction.
+            // ParticleBatcher is constructed post-ctor by WbMeshAdapter (WbMeshAdapter.cs:78)
+            // after the adapter has wired up all dependencies. The null! here is overridden
+            // immediately after construction; it is not observable as null at runtime.
             ParticleBatcher = null!;
         }