On song:loaded, an optional pack intro plays once: a camera flyover video
(idle layer, one-shot) with bar-crowd ambience audio that ducks out on
song:play, near the flyover's landing, or at handoff — whichever first.
Machine commits and stingers defer during the intro; stop()/song-change
abort it. Packs without an intro behave as before.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>