Audio-only mpv core on every platform (dedicated
com.plezy/mpv_audio_player channels): parameterized android/windows/
linux mpv plugins and a new apple MpvAudioPlayerCore, all skipping
video/window paths (vid=no, audio-display=no, gapless-audio=weak).
MusicPlaybackService drives an in-memory queue with shuffle/repeat,
file-loaded-event gapless arming (property edges coalesce and the
android bridge drops them), per-track progress reporting, OS media
controls, audio focus, sleep timer, and error auto-skip.
PlaybackCoordinator enforces one live native player: starting video
disposes the audio core first.
close#1453
Events were delivered to the Flutter event sink directly on the mpv event
thread while method replies were already marshaled; concurrent messenger
use from two threads intermittently crashed guests during the event burst
of an in-place media switch. Also joins the event thread before draining
platform tasks in the plugin destructor (reverse member destruction order
would otherwise let the event thread enqueue into a destroyed queue) and
coalesces the wakeup PostMessage behind a retry-safe flag.