Add missing stickman placeholder and stage scene I/O functionality

- Implemented `MissingStickmanPlaceholder` class to visually indicate missing stickman assets in the scene.
- Created `StageSceneIO` class for handling JSON serialization and deserialization of stage documents, including encoding and decoding of actions and rules.
- Added tests for vector handling, rule encoding/decoding, safe file writing, root validation, and handling of missing assets.
- Introduced a new stickman asset file `bad.stk` for testing purposes.
- Enhanced test coverage for phase 5 file features, ensuring robust handling of scene data and missing assets.

scripts/stickman_rig.gd

@@ -478,6 +478,9 @@ var _spawn_limb_rotations: Dictionary = {}
 
 var action_queue: Array[Dictionary] = []
 
+## Original .stk asset path, assigned by StageSpawner for scene persistence.
+var source_stk_path: String = ""
+
 var _runner_state: RunnerState = RunnerState.IDLE
 var _action_phase: ActionPhase = ActionPhase.NONE
 var _current_index: int = -1