fix: site player now also pauses fallback embeds (mutual exclusion both ways)
The registry already handled mutual exclusion for custom players (API loaded),
but fallback iframes (ad-blocker blocks the API) didn't register themselves:
the site player couldn't stop them -> double audio. A cross-origin iframe
can't be paused via API, so we now register the fallback in the registry with
a pause() that reloads the iframe WITHOUT autoplay (= stops it, remains
restartable). Result: starting an embed pauses the site player, and starting
the site player stops the embed. embed-player.js v=3 -> v=4.
Co-Authored-By: Claude <noreply@…>