refactor(M03): extract M2 build batch planner

src/render/m2/m2_build_batch_planner.gd

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+class_name M2BuildBatchPlanner
+extends RefCounted
+
+## Stateless M2 build batch sizing and group-cursor progression.
+
+
+## Plans one static or animated build batch. The returned Dictionary is detached
+## and contains effective_batch_size, batch_count, next_offset and group_complete.
+func plan_batch(
+		transform_count: int,
+		current_offset: int,
+		has_animated_prototype: bool,
+		animated_batch_limit: int,
+		static_batch_limit: int
+) -> Dictionary:
+	var selected_batch_limit := (
+		animated_batch_limit if has_animated_prototype else static_batch_limit
+	)
+	var effective_batch_size := maxi(1, selected_batch_limit)
+	var remaining_transform_count := maxi(0, transform_count - current_offset)
+	var batch_count := mini(effective_batch_size, remaining_transform_count)
+	var is_group_complete := (
+		batch_count <= 0 or current_offset + batch_count >= transform_count
+	)
+	return {
+		"effective_batch_size": effective_batch_size,
+		"batch_count": batch_count,
+		"next_offset": 0 if is_group_complete else current_offset + batch_count,
+		"group_complete": is_group_complete,
+	}

src/render/m2/m2_build_batch_planner.gd.uid

@@ -0,0 +1 @@
+uid://d0my7yntfsvuy