feat: Integrate custom prop library and sandbox functionality

- Added support for custom props in the sandbox, including loading, saving, and rendering.
- Implemented `PropLibrary` for managing custom prop entries and scanning for `.prop` files.
- Introduced `PropVisuals` for shared visual and collision representation of custom props.
- Enhanced `StageSpawner` to handle spawning of custom props with layered visuals and collision hulls.
- Created `MissingPropPlaceholder` to manage missing custom props during scene reloads.
- Updated thumbnail generation to support custom props with unique keys based on file paths.
- Added tests for custom prop lifecycle, authored visual values, and missing prop handling.
- Refactored existing code to accommodate new prop functionalities and ensure backward compatibility.

scripts/stage_selection.gd

@@ -141,6 +141,12 @@ static func get_world_aabb(node: Node2D) -> Rect2:
 	# Duck-typed TriggerArea: expose its centered local rect through the transform.
 	if node.has_method("get_area_rect"):
 		return node.global_transform * (node.call("get_area_rect") as Rect2)
+	var visuals := node.get_node_or_null(NodePath("Visuals")) as Node2D
+	if visuals != null:
+		var visual_acc := { "min_x": INF, "min_y": INF, "max_x": -INF, "max_y": -INF }
+		_collect_visual_points(visuals, visual_acc)
+		if visual_acc["min_x"] <= visual_acc["max_x"]:
+			return Rect2(Vector2(visual_acc["min_x"], visual_acc["min_y"]), Vector2(visual_acc["max_x"] - visual_acc["min_x"], visual_acc["max_y"] - visual_acc["min_y"]))
 	var poly := node.get_node_or_null(NodePath("Polygon2D")) as Polygon2D
 	if poly != null and not poly.polygon.is_empty():
 		var rect := Rect2(node.to_global(poly.polygon[0]), Vector2.ZERO)