The audio-only core reused the video core's property registrations
wholesale, so every music session observed track-list,
demuxer-cache-state, audio-device-list, audio-device, secondary-sid,
seekable, paused-for-cache, volume, speed, aid and sid. libmpv sends an
initial notification plus an event per edge for each one, and three of
them decode structured nodes into track and device models nothing on the
music path reads.
Register time-pos, duration, pause, eof-reached and playlist-pos there;
the video path keeps the full set. Also drop the post-initialization
`gapless-audio` write: every native audio core already sets it before
mpv_initialize, which is where libmpv wants initial configuration.
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