[nooga]

The SM-24 Mono sounded like a 303. Its HOT/WET lowpass subtracts a clipped lowpass at a hot input, so PEAK cut the bass by 16–25 dB and put the peak 1.5–2 octaves above CUT. This PR gives the SM-24 the MS-20's own filters and a patch panel, and moves the acid filter into its own synth.

SM-24 Mono: K35 and OTA filters (3c41640)

• LPF MODE gains K35 and OTA, built on Stinchcombe's MS-20 transfer function.
  • Both run HPF → LPF in series and resonate up to self-oscillation.
  • The PEAK pot taper is ours: it gives +6 dB of peak at noon.
• K35 is the new default.
• HOT and WET stay for the acid sound.
• Measured:
  • bass stays within ±0.4 dB at any PEAK;
  • the peak sits within 0.02 octave of CUT;
  • slope is −11 to −12 dB/oct;
  • K35 distorts lopsided, OTA symmetric.
• New presets: k35-scream, k35-round-bass, ota-singing-lead.
• The init patch is re-voiced. Saved songs keep their mode.

Squelch 33, a new acid synth (8f10ce1)

• Built on the HOT/WET filter (ms20_lpf.fy), which is not changed.
• A mono 303-style voice:
  • SAW or SQR oscillator;
  • decay-only filter envelope with ENV MOD;
  • accent from velocity: a 200 ms envelope, a VCA push, and an accent RC that stacks over consecutive accents;
  • slide on overlapping notes.
• Measured:
  • accent +4–5 dB and 20–40% brighter;
  • RES thins the fundamental 13 dB;
  • presets level at −20 dB RMS ±0.4.
• Presets: classic-squelch, square-bounce, wet-scream, round-line.

SM-24 Mono: patch panel (d9f0abe)

• Four cords evaluated every sample, with each source arriving one sample late. That delay allows feedback such as VCA OUT → FLT IN.
• Uses Concoction's dock, knob rings and matrix display, on a new PATCH page.
• An unpatched SM-24 renders bit-exactly as before.
• VCO1's wave became a select. This keeps the bay within fy's 32 versioned bodies; without it, an unpatched voice cost 609 ns per sample.
• Five patch-* presets: S&H bubbles, feedback growl, FM metal bass, tracking lead, vowel sweep.

Demos (068d58c, d9f0abe)

• songs/k35_night_run.py
• songs/squelch_acid.py
• songs/ms20_patch_bay.py

Checks

• zig build test: 527/527 pass.
• zig build bench -- --all --check --no-sheets: 174 golden cases are bit-exact. ms20 goldens were re-recorded in 3c41640 for the new default; acid goldens are new.
  • The command still exits 1, because shift2, resonator2, buffer2 and phaser2 have never had goldens. That predates this branch.
• The PATCH page hasn't been checked by hand in the app.

Docs: docs/14 (new sections with measurements), docs/20 (regenerated), CHANGELOG.

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