Choreograph: add required param validation, teach parameters in tutorial

- Add required parameter validation: scenes abort with error if params without defaults are missing
- Fix: don't show status card when executeScene returns null (failed to start)
- variables-and-templates: add parameter teaching steps (required vs default)
- variables-and-templates: fire breath row uses speed param instead of hardcoded 0.2
- Add castName to scope for use in command templates