Three events the frontend had no way to learn about:
- `PlaybackPositionChanged` carries `player.PositionInfo`, so the seek
bar can render what the player is doing instead of counting seconds
itself.
- `PlaybackFailed` carries the file and the reason, from both the load
and the play path, so a track that will not play stops being a
silent no-op.
- `TrackPlayCountChanged` carries everything needed to patch one track
in place. `TrackMetadataChanged` means "the tags on disk were
rewritten" and costs the frontend its entire library cache; finishing
a track used to emit it.
An event's cost is part of its meaning, and the expensive one must not
be reused for something cheap.
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