Holding a chord on an arpeggiated channel used to collapse to the last (rightmost)
note and retrigger only that — the other notes were killed by the per-note
releaseBus + single-note arp.start. ChannelArp now holds the whole chord (sorted,
de-duped) and cycles all its notes across the octave range in the chosen mode.
Notes entered on the same line accumulate into one chord via a per-line generation
stamp; a new line starts a fresh chord, and a looping chord keeps its running phase
so it doesn't glitch at the loop point. ChannelArp is made internal to cover the
sequencing with ChannelArpTest.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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