Voice signal flow and performance features for SH-101 polyphony

[seclorum]

Restructure the signal flow and add performance features to match SH-101 per-voice architecture, with polyphony:

• Ensure signal flow: VCO+Noise+Sub → Mixer → VCF → VCA per voice.
• Add portamento/glide per voice (exponential/linear selectable).
• Maintain SIMD optimization and ensure performance with new features.
• Refactor MIDI handling to support accent, legato, and polyphonic performance.
• Update relevant code areas:
  • Source/PluginProcessor.h
  • Source/PluginProcessor.cpp

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