refactor(M03): extract WMO render build planner

src/render/wmo/wmo_render_build_step_planner.gd

@@ -0,0 +1,40 @@
+class_name WmoRenderBuildStepPlanner
+extends RefCounted
+
+## Selects one value-only WMO render-group build operation without engine objects.
+
+const OPERATION_MESH: StringName = &"mesh"
+const OPERATION_MULTIMESH: StringName = &"multimesh"
+const OPERATION_COMPLETE: StringName = &"complete"
+
+
+## Selects the next mesh-first build operation and returns its selected index
+## plus the cursor values to adopt after that one operation. Counts and cursors
+## are intentionally not clamped so the loader's historical comparisons remain
+## exact for every integer input.
+func plan_step(
+		mesh_count: int,
+		mesh_index: int,
+		multimesh_count: int,
+		multimesh_index: int
+) -> Dictionary:
+	if mesh_index < mesh_count:
+		return {
+			"operation": OPERATION_MESH,
+			"selected_index": mesh_index,
+			"next_mesh_index": mesh_index + 1,
+			"next_multimesh_index": multimesh_index,
+		}
+	if multimesh_index < multimesh_count:
+		return {
+			"operation": OPERATION_MULTIMESH,
+			"selected_index": multimesh_index,
+			"next_mesh_index": mesh_index,
+			"next_multimesh_index": multimesh_index + 1,
+		}
+	return {
+		"operation": OPERATION_COMPLETE,
+		"selected_index": -1,
+		"next_mesh_index": mesh_index,
+		"next_multimesh_index": multimesh_index,
+	}