feat: notepat F8 hold/latch — sustain currently-playing notes
F8 captures all currently-held keys and keeps them sounding after
key release. Press F8 again to release all held notes. Visual
HOLD indicator shows next to octave in the wave row.
- High beep on engage, low beep on release
- Held keys immune to key-up until F8 toggled off
- stopAllSounds clears hold state (escape exit, etc.)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>