feat(music): mpv audio playback engine with gapless queue service

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.

ios/Runner/AppDelegate.swift

@@ -36,6 +36,11 @@ import MediaPlayer
       MpvPlayerPlugin.register(with: registrar)
     }
 
+    // Register the audio-only MPV player plugin (music playback)
+    if let registrar = engineBridge.pluginRegistry.registrar(forPlugin: "MpvAudioPlayerPlugin") {
+      MpvAudioPlayerPlugin.register(with: registrar)
+    }
+
     if let registrar = engineBridge.pluginRegistry.registrar(forPlugin: "AtmosProbePlugin") {
       AtmosProbePlugin.register(with: registrar)
     }