Replaces the single-root-pattern sequencer with a Track[] array that
allows multiple patterns to loop independently. Adds ADD_TRACK (0x0A),
REMOVE_TRACK (0x0B), PLAY_TRACKS (0x0C), and SET_TRACK_MUTE (0x0D)
protocol records. The C backend gains per-track pending_subs and a
tracks_mutex. The Node server gains track-state APIs (/api/tracks/:id/
active, mute, solo) and the frontend shows per-pattern track/mute/solo
buttons in the sidebar list.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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