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;