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.

linux/runner/mpv/mpv_plugin.h

@@ -9,21 +9,26 @@
 
 G_BEGIN_DECLS
 
-/// Plugin for MPV video playback on Linux.
+/// Plugin for MPV playback on Linux.
 ///
-/// This plugin renders mpv video through Flutter's GPU-accelerated
-/// texture pipeline via FlTextureGL.
+/// The video instance renders mpv video through Flutter's GPU-accelerated
+/// texture pipeline via FlTextureGL. The audio-only instance (music
+/// playback) skips all texture/GL work and runs mpv with video disabled.
 
 #define MPV_PLUGIN_TYPE (mpv_plugin_get_type())
 
 G_DECLARE_FINAL_TYPE(MpvPlugin, mpv_plugin, MPV, PLUGIN, GObject)
 
-/// Creates a new MpvPlugin instance.
-MpvPlugin* mpv_plugin_new(FlPluginRegistrar* registrar);
+/// Creates a new MpvPlugin instance on the given method channel name (the
+/// event channel is |channel_name| + "/events").
+MpvPlugin* mpv_plugin_new(FlPluginRegistrar* registrar, const gchar* channel_name, gboolean audio_only);
 
-/// Registers the plugin with Flutter.
+/// Registers the video plugin with Flutter.
 void mpv_plugin_register_with_registrar(FlPluginRegistrar* registrar);
 
+/// Registers the audio-only (music) plugin with Flutter.
+void mpv_audio_plugin_register_with_registrar(FlPluginRegistrar* registrar);
+
 G_END_DECLS
 
 #endif  // MPV_PLUGIN_H_