Music on iOS played through a mixable audio session: mpv's audiounit
output requests mixWithOthers unless audio-exclusive is set, and a
mixable session disqualifies the app from Now Playing. iOS ignored the
published metadata and remote-command targets, so the lock screen showed
no controls, headphone buttons drove the previous media app, and other
apps kept playing alongside plezy.
Set audio-exclusive on the audio-only core at init on iOS — the same
contract the video player already applies at playback start. Its only
effect there is dropping mixWithOthers.
close#1921
edde746
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