Add sandbox stage builder scripts and functionality

- Introduced StageGizmos for hover highlighting, selection outlines, and rotation handles in the sandbox stage builder.
- Added StageGrid for an optional world-space grid overlay that adjusts with camera panning and zooming.
- Implemented StageSelection for geometric hit-testing and selection management of nodes in the sandbox.
- Created StageSpawner as a registry-driven factory for spawning terrain, props, and stickmen, allowing for dynamic template management.
- Each script includes necessary constants, state management, and public API methods for interaction.

scripts/stk_rig_adapter.gd

@@ -233,9 +233,14 @@ static func _mount_shapes(stk_data: Dictionary, rig: Node2D) -> void:
 		var part_data: Variant = body_parts.get(part_name, {})
 		if part_data is Dictionary:
 			var pd := part_data as Dictionary
-			var shapes_var: Variant = pd.get("shapes", [])
-			if shapes_var is Array:
-				shapes = shapes_var as Array
+			if pd.has("shapes"):
+				var shapes_var: Variant = pd.get("shapes")
+				if shapes_var is Array:
+					shapes = shapes_var as Array
+			elif pd.has("points"):
+				# v1.0/v1.1 single-shape format: the part dict is itself one
+				# shape (wrapped like the editor's load path).
+				shapes = [pd]
 			# Phase 9 Round 3: the preview's per-part rotation (degrees) and
 			# scale about the bbox center are applied to the mounted geometry.
 			rotation_deg = float(pd.get("rotation", 0.0))