refactor(M03): extract M2 placement grouper

src/render/m2/m2_placement_grouper.gd

@@ -0,0 +1,65 @@
+class_name M2PlacementGrouper
+extends RefCounted
+
+## Stateless ADT M2 placement validation, normalization and transform grouping.
+
+const M2_PLACEMENT_TRANSFORM_RESOLVER_SCRIPT := preload(
+	"res://src/render/m2/m2_placement_transform_resolver.gd"
+)
+
+
+## Groups valid placements by normalized relative M2 path while preserving
+## placement order inside each group. Returned containers are caller-owned.
+func group_placements(
+		tile_origin: Vector3,
+		m2_names: PackedStringArray,
+		m2_placements: Array
+) -> Dictionary:
+	var groups: Dictionary = {}
+	var placement_transform_resolver := (
+		M2_PLACEMENT_TRANSFORM_RESOLVER_SCRIPT.new()
+	)
+	for placement_variant in m2_placements:
+		if not (placement_variant is Dictionary):
+			continue
+		var placement: Dictionary = placement_variant
+		var name_id: int = int(placement.get("name_id", -1))
+		if name_id < 0 or name_id >= m2_names.size():
+			continue
+
+		var normalized_relative_path := _normalize_relative_path(m2_names[name_id])
+		if normalized_relative_path.is_empty():
+			continue
+
+		var world_position: Vector3 = placement.get("pos", Vector3.ZERO)
+		var local_position := world_position - tile_origin
+		var rotation_radians: Vector3 = placement.get("rot", Vector3.ZERO)
+		var scale_value := float(placement.get("scale", 1.0))
+		var placement_basis: Basis = placement_transform_resolver.resolve_basis(
+			rotation_radians,
+			normalized_relative_path
+		)
+		var origin_offset: Vector3 = placement_transform_resolver.resolve_origin_offset(
+			rotation_radians,
+			normalized_relative_path,
+			scale_value
+		)
+		var local_transform := Transform3D(
+			placement_basis.scaled(Vector3.ONE * maxf(scale_value, 0.0001)),
+			local_position + origin_offset
+		)
+
+		if not groups.has(normalized_relative_path):
+			groups[normalized_relative_path] = []
+		(groups[normalized_relative_path] as Array).append(local_transform)
+	return groups
+
+
+func _normalize_relative_path(relative_path: String) -> String:
+	var normalized_relative_path := relative_path.replace("\\", "/")
+	if (
+		normalized_relative_path.ends_with(".mdx")
+		or normalized_relative_path.ends_with(".mdl")
+	):
+		normalized_relative_path = normalized_relative_path.get_basename() + ".m2"
+	return normalized_relative_path