Implement SH-101-style architecture for each polyphonic voice

[seclorum]

This feature request is to refactor and enhance the synth engine so each polyphonic voice closely models the signal path and features of the Roland SH-101, but with modern polyphony. The goal is to make each voice a true SH-101 channel, while retaining SIMD optimizations.

Scope of changes:

Oscillator Section

• Add noise generator (white and pink noise) as selectable source or blend with main VCO.
• Implement pulse waveform for main oscillator with variable pulse width and pulse width modulation (PWM).
• Ensure sub-oscillator is always square wave and strictly -1 or -2 octaves below main VCO (with selectable range).
• Main oscillator should support saw, square/pulse, and noise, selectable or blendable per SH-101 style.

Filter Section

• Refactor per-voice filter to model IR3109/SH-101 characteristics: 24dB lowpass, analog non-linearity, resonance with self-oscillation, key tracking, and variable envelope modulation amount.
• Add option for filter envelope amount and keyboard tracking per voice.
• Support filter self-oscillation at high resonance.

Modulation and Envelope Section

• Ensure each voice has two independent envelope generators: one for VCA, one for VCF (per SH-101).
• LFO should offer triangle and square waveforms, modulate VCO pitch, PWM, and VCF cutoff independently.
• Add modulation matrix or routing logic for LFO/envelope destinations per SH-101 panel.

Other Features

• Add portamento/glide per voice (exponential/linear selectable).
• Add accent (velocity-sensitive envelope) and proper MIDI legato handling for polyphony.
• Restructure signal flow so each voice follows: VCO+Noise+Sub → Mixer → VCF → VCA.
• Maintain SIMD optimization and polyphonic capabilities.

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Example code areas for modification:

• Source/PluginProcessor.h: Extend Voice struct with noise, PWM, dedicated envelopes, routing flags, accent, and portamento params.
• Source/PluginProcessor.cpp: Refactor oscillator, filter, and envelope logic for signal path and feature changes.
• lab/simdsynth.cpp: Prototype new oscillator and filter behaviors.

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This issue is for a major feature refactor and should be split into sub-issues:

• Oscillator/Noise/PWM refactor
• Filter model upgrade
• Envelope/LFO/Modulation matrix
• Voice signal flow and performance features

Label: enhancement

See the following code references:

• Voice struct definition
• Oscillator signal generation
• Envelope and filter handling

[seclorum]

Good work, Claude.