Oscillator/Noise/PWM refactor for SH-101-style voice

[seclorum]

Implement the SH-101-style oscillator section for each polyphonic voice.

• Add selectable noise generator (white/pink) as an additional source or blendable with VCO.
• Implement pulse waveform with variable pulse width and pulse width modulation (PWM) for the main oscillator.
• Restrict sub-oscillator to square wave, strictly -1 or -2 octaves below main VCO, with selectable range.
• Ensure main oscillator supports saw, square/pulse, and noise sources, selectable or blendable per voice.
• Update oscillator signal generation logic to match SH-101 signal flow.
• Update relevant code areas:
  • Source/PluginProcessor.h
  • Source/PluginProcessor.cpp
  • lab/simdsynth.cpp

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