- 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.
