feat: improved kick voice, synth engine features, CPU optimizations, …

[illobo]

…bump v2.0.3

Kick voice improvements:

• Dual-stage pitch envelope (fast 2.5ms attack + color-controlled settle)
• BP/LP click blend for 909 bridged-T "knock" character
• Subharmonic at 0.5× (octave below) replacing 0.75× fourth
• Asymmetric drive saturation for even-harmonic analog warmth
• 0.5ms attack ramp to prevent DC click artifact
• Drive-dependent 2nd harmonic (10-45% based on drive)
• Retuned all kick presets and defaults for new voice

Synth engine features:

• Portamento/glide with exponential pitch smoothing (5-500ms)
• Filter key follow (note pitch modulates cutoff relative to C4)
• Sub oscillator waveform choice (Square/Sine/Saw)
• Hard osc sync (osc2 phase reset on osc1 cycle)
• 4-voice supersaw with wider spread (up to 50 cents)
• UI controls: Glide slider, [X]Sync toggle, Sub:[] selector, KeyFl slider

CPU optimizations (no quality loss):

• Cached SVF filter g/k coefficients (skip tan() when unchanged)
• Fast exp2 polynomial for pitch ratios (replace per-sample powf)
• Incremental peak tracking in lookahead limiter (amortized O(1))
• Skip inactive oscillators (osc1/osc2/sub when level=0 or env idle)