Filter model upgrade to SH-101/IR3109 style

[seclorum]

Refactor the per-voice filter to emulate the Roland SH-101 (IR3109) characteristics:

• 24dB/oct lowpass topology with analog non-linearity and resonance, including self-oscillation behavior.
• Add filter envelope modulation amount and keyboard tracking control per voice.
• Allow filter self-oscillation at high resonance settings.
• Update filter signal path and modulation logic to match SH-101.
• Update relevant code areas:
  • Source/PluginProcessor.h
  • Source/PluginProcessor.cpp
  • lab/simdsynth.cpp

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