Input moves from per-key gestures to one octave-level multi-touch handler in
PianoOctave, so a finger can slide across keys and each note retriggers as it
crosses a key boundary (black keys win in their upper region via geometry-aware
hit-testing); multiple fingers still play independently for chords. Out-of-scale
keys are skipped mid-slide. onNoteOn carries a `slid` flag so the tracker
punch-in keyboard records only the initial press (a slide auditions a glide
without spraying a run of notes into the pattern), while the toolbox / editor
audition keyboards play every key crossed. StackedKeyboard and SlotAuditionKeyboard
both migrate to the new callback API; PianoOctave now renders the keys itself.
Verified on-device: a slow slide lights each in-scale key as the finger reaches
it and skips out-of-scale keys.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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