Choreograph: add onSceneStart/onSceneFinish signals, delay completion card

- Add signal/slot system: Choreograph.onSceneStart(fn) and Choreograph.onSceneFinish(fn)
  Both return unsubscribe functions. Listeners receive { name, instanceId, cast, params }.
- Delay completion card by 500ms so it doesn't step on the last command output
- Use signals in tutorial step 6 to auto-advance after scene finishes
  (replaces polling approach)