Choreograph: tutorial polish and waitForScene helper

- Add Choreograph.waitForScene(name) public helper (signal-based, replaces polling)
- Replace all inline onSceneStart/onSceneFinish blocks with ...waitForScene()
- filters-and-delay: use explode-death FX instead of broken emoji echo
- filters-and-delay: convert step 7 to raw HTML, focus on demonstrated concepts
- filters-and-delay: add dynamic help handout links with anchors
- your-first-scene: require min 4 tokens selected (ScriptKit select gate)
- Fix duplicate closing brace syntax error in roles-and-casts