render: extract static M2 batch materializer

docs/modules/m2-mesh-resource-cache-state.md

@@ -36,7 +36,7 @@ flowchart LR
     IO --> Extract[M2MeshResourceExtractor]
     Extract --> Prepare[M2RuntimeMeshFinalizer]
     Prepare --> Cache
-    Loader --> Build[M2 MultiMesh materialization]
+    Loader --> Build[M2StaticBatchMaterializer]
 ```
 
 The cache may retain `Mesh` resources and copied normalized-path Strings. It has
@@ -118,7 +118,8 @@ flowchart TB
     Loader[StreamingWorldLoader] --> Cache[M2MeshResourceCacheState]
     Loader --> Pipeline[M2MeshLoadPipelineState]
     Loader --> Finalizer[M2RuntimeMeshFinalizer]
-    Loader --> Builder[M2Builder and MultiMesh]
+    Loader --> Builder[M2Builder]
+    Loader --> Materializer[M2StaticBatchMaterializer]
     Cache --> Mesh[Godot Mesh Resource]
     Cache -. no dependency .-> Pipeline
     Cache -. no dependency .-> Classifier
@@ -132,7 +133,8 @@ flowchart TB
 - Borrowed Mesh lookups do not transfer ownership or duplicate resources.
 - `M2MeshResourceExtractor` owns first-Mesh selection and temporary PackedScene
   instances. `M2PrototypeCacheState` owns missing/prototype/animated state; the
-  loader owns MultiMeshes and adoption decisions.
+  static materializer owns MultiMesh construction/attachment; the loader owns
+  resource adoption and build-job decisions.
 - The loader drains asynchronous work before the final cache clear.
 
 ## Errors, cancellation and recovery