On macOS 27 beta, CoreAudio rejects ao_coreaudio's channel-layout setup
with paramErr (-50), and since 2.14.0 pinned ao=coreaudio as the only
output, the failed init left every video playing with no audio and no
selectable audio track. Append avfoundation as the fallback, mirroring
upstream mpv's macOS probe order: every format still opens through the
HAL-backed CoreAudio path when it works, and the fallback only engages
when CoreAudio's init fails outright.
close#1964
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