Envelope, LFO, and Modulation Matrix for SH-101-style voice #4
Description
Refactor and extend the modulation and envelope section for each polyphonic SH-101-style voice:
- Two independent envelope generators per voice (VCA and VCF).
- LFO with triangle and square waveforms; modulate VCO pitch, PWM, and VCF cutoff independently.
- Add modulation routing/matrix logic per voice to allow classic SH-101 modulation paths (panel-style destinations).
- Implement accent (velocity-sensitive envelope) and proper MIDI legato handling for polyphony.
- Update relevant code areas:
Label: enhancement
Sub-issue of: sh101-architecture-polyphonic