Highlight a cell on the arrangement roll and Play now begins at that selection's
first beat instead of always from the top. Stop clears the highlight, so the next
Play resumes from the beginning. The rectangular selection is hoisted from
ArrangementRoll to the ViewModel (four ints) so the transport can read/clear it;
engine.play() gains a startBeat that seeks playlistIndex in the built playlist
(A/B expansion respected; falls back to the top if the beat isn't in it) or
currentLine in pattern-loop mode. Loading/importing a project also clears the
selection so a stale one can't apply to a new arrangement.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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