Culture Feed unmuted autoplay audio (P0)

Priority: P0. Scrolling the Culture Feed started videos with sound with no user tap, and once a video was running there was no way to stop the sound.

Fix: the fallback embed no longer autoplays and now unmounts on scroll-off, the sticky unmute preference has been removed, and a new audio controller enforces at most one audible element at a time and mutes anything leaving the viewport.

Verification: 19/19 tests and 16/16 browser assertions passing on both branches, CI green, no regressions.

State: deployed to dev and QA.