phase(N.4): AcSurfaceMetadata side-table for WB-pristine surface props

Holds Translucency / Luminosity / Diffuse / SurfOpacity / NeedsUvRepeat /
DisableFog keyed by (gfxObjId, surfaceIdx). Populated at extraction time,
queried by the draw dispatcher. ConcurrentDictionary because mesh
extraction happens on background workers.

No fork patches required — keeps WB's MeshBatchData pristine.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,21 @@
+using AcDream.Core.Meshing;
+
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// AC-specific surface render metadata that WB's <c>MeshBatchData</c>
+/// doesn't carry. Computed at mesh-extraction time and looked up by the
+/// draw dispatcher to drive translucency / sky-pass / fog behavior.
+///
+/// <para>
+/// All fields mirror those on today's <see cref="GfxObjSubMesh"/> so
+/// behavior is preserved bit-for-bit through the migration.
+/// </para>
+/// </summary>
+public sealed record AcSurfaceMetadata(
+    TranslucencyKind Translucency,
+    float Luminosity,
+    float Diffuse,
+    float SurfOpacity,
+    bool NeedsUvRepeat,
+    bool DisableFog);