The seek bar was a setInterval counter reconciled only on track
change: measured 3 s behind during steady playback and 30 s behind
after four keyboard seeks, because the seek shortcut never told it.
And `loadCurrentTrack`/`playCurrentTrack` logged, returned false and
emitted nothing, so double-clicking a moved file did nothing, twice,
forever — while auto-advance onto a bad file stopped playback dead.
- A 1 Hz position ticker while playing, plus an immediate report on
load, play, pause, seek and natural finish. The payload carries a
`trackChangeId` (the store is a singleton, so a bar mounting later
must not adopt a report about the previous track) and a `seq` (the
same second reported twice still has to reset interpolation).
- `PlaybackFailed` from both failure paths, and `playCurrentOrSkip`
steps over tracks that will not load — bounded by the queue length,
so a disconnected drive stops after one pass instead of spinning
through a RepeatAll wrap. `PlayIndex` still reverts: the user picked
that track.
- `SeekFailed` is emitted when the seek itself fails, not only when
nothing is loaded, and is followed by a position report so the
optimistic move is taken back by the mechanism that fixed the drift.
- A queue that simply ran out no longer unloads the player, so the
finished track stays on the bar at 0:00.
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