Envelope, LFO, and Modulation Matrix for SH-101-style voice

[seclorum]

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:
  • Source/PluginProcessor.h
  • Source/PluginProcessor.cpp

Label: enhancement
Sub-issue of: sh101-architecture-polyphonic