render: extract static M2 batch materializer

src/render/m2/m2_static_batch_materializer.gd

@@ -0,0 +1,56 @@
+class_name M2StaticBatchMaterializer
+extends RefCounted
+
+## Builds and attaches one static M2 MultiMesh batch from an ordered transform
+## slice. All Resource and SceneTree mutation belongs on the main thread.
+
+
+## Returns the attached MultiMeshInstance3D, or null for invalid/empty input.
+## The supplied parent owns the returned node and its MultiMesh Resource.
+## Transform bounds are a build-planner precondition and are not clamped.
+func materialize_batch(
+	m2_parent_root: Node3D,
+	relative_path: String,
+	mesh: Mesh,
+	transforms: Array,
+	start_index: int,
+	instance_count: int,
+	batch_serial: int,
+	visibility_range_end: float,
+	visibility_range_margin: float,
+	cast_shadows: bool
+) -> MultiMeshInstance3D:
+	if (
+		m2_parent_root == null
+		or mesh == null
+		or transforms.is_empty()
+		or instance_count <= 0
+	):
+		return null
+
+	var multimesh := MultiMesh.new()
+	multimesh.transform_format = MultiMesh.TRANSFORM_3D
+	multimesh.mesh = mesh
+	multimesh.instance_count = instance_count
+	for batch_offset in instance_count:
+		multimesh.set_instance_transform(
+			batch_offset,
+			transforms[start_index + batch_offset]
+		)
+
+	var multimesh_instance := MultiMeshInstance3D.new()
+	multimesh_instance.name = "%s_%d" % [
+		relative_path.get_file().get_basename(),
+		batch_serial,
+	]
+	multimesh_instance.multimesh = multimesh
+	multimesh_instance.cast_shadow = (
+		GeometryInstance3D.SHADOW_CASTING_SETTING_ON
+		if cast_shadows
+		else GeometryInstance3D.SHADOW_CASTING_SETTING_OFF
+	)
+	if visibility_range_end > 0.0:
+		multimesh_instance.visibility_range_end = visibility_range_end
+		multimesh_instance.visibility_range_end_margin = visibility_range_margin
+	m2_parent_root.add_child(multimesh_instance)
+	return multimesh_instance