Show a Saturation node as a device that glows with its drive (#396)

A Saturation in the graph was a stack of rows. The user wanted a device
of its own style and size, like the Delay and the Reverb, that shows
warmth and heat the harder the sound is driven.

- The node is a copper plate of 720x280 with a button per type, the
  auto-gain lever, a big drive knob and small knobs for tone, output
  and wet. Each change is one undo step, wired numbers lock their knob.
- The screen draws a sine at -12 dBFS, the level auto-gain is reckoned
  at, as a grey ghost. Over it is the wave that comes out: bent by the
  type, its DC taken out, auto-gain and output after it, blended by
  wet. Under it stand the auto-gain and the peak.
- The heat is the drive over 48 dB, as far as wet lets it be heard.
  The wave goes from cold steel through ember red to white, a glow
  rises from below and flickers, and embers fly up from about 8 dB on.
  A lamp beside the name glows with it.
- The node keeps the editor drawing only while there is heat. Without
  drive it draws nothing, unlike the Delay, which always animates.
- The Reverb's private type button is now PickButton with its own
  colours, and the Saturation's type buttons use it as well.
- No instrument preset uses a Saturation, so no preset needs more
  columns.

Tests:
- Unit saturation-view: 14 new tests.
- Counter-check: without taking out the DC, exactly the DC test fails.
- e2e 34 (atx): ALL PASSED, 137 checks, one new for type, lever and
  drive with undo.
- Gate (atx): CHECK EXIT 0, 289 files, 4431 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/soundfx/nodes.ts

@@ -1095,15 +1095,15 @@ export const SoundFxLimiter: SoundFxNodeDef = effect(
 const DRIVE = { ...numberRow('drive', 'Drive (dB)', 0, 48, 0.5), default: 12 };
 const TONE = { ...numberRow('tone', 'Tone', 0, 1, 0.01), default: 1 };
 const OUTPUT = numberRow('output', 'Output (dB)', -24, 24, 0.5);
-const SATURATION_AUTO_GAIN = { ...COMPRESS_AUTO_GAIN, default: true };
+export const SATURATION_AUTO_GAIN = { ...COMPRESS_AUTO_GAIN, default: true };
 const SATURATION_MIX = { ...numberRow('mix', 'Wet', 0, 1, 0.01), default: 1 };
-const SATURATION_ROWS = {
+export const SATURATION_ROWS = {
   drive: DRIVE,
   tone: TONE,
   output: OUTPUT,
   mix: SATURATION_MIX,
 };
-const SATURATION_TYPE = {
+export const SATURATION_TYPE = {
   key: 'curve',
   label: 'Type',
   kind: 'enum' as const,
@@ -1111,7 +1111,7 @@ const SATURATION_TYPE = {
   options: SATURATION_TYPES,
 };
 
-function saturationTypeOf(settings: PropertyBag): SaturationType {
+export function saturationTypeOf(settings: PropertyBag): SaturationType {
   return SATURATION_TYPES.find((option) => option.value === settings[SATURATION_TYPE.key])?.value ?? 'soft';
 }
 

src/core/soundfx/saturation.ts

@@ -71,7 +71,7 @@ export function saturationCurve(type: SaturationType): Float32Array {
 }
 
 /** The level a reference sine is at: -12 dBFS, about where a sound is loud but not at its peak. */
-const REFERENCE = 0.25;
+export const SATURATION_REFERENCE = 0.25;
 const STEPS = 256;
 
 /**
@@ -82,11 +82,11 @@ const STEPS = 256;
 export function saturationAutoGain(type: SaturationType, driveDb: number): number {
   const drive = 10 ** (driveDb / 20);
   const out = Array.from({ length: STEPS }, (_, i) =>
-    saturate(type, REFERENCE * Math.sin((2 * Math.PI * i) / STEPS) * drive),
+    saturate(type, SATURATION_REFERENCE * Math.sin((2 * Math.PI * i) / STEPS) * drive),
   );
   const mean = out.reduce((sum, v) => sum + v, 0) / STEPS;
   const rms = Math.sqrt(out.reduce((sum, v) => sum + (v - mean) ** 2, 0) / STEPS);
-  const gain = rms > 0 ? 20 * Math.log10(REFERENCE / Math.SQRT2 / rms) : 48;
+  const gain = rms > 0 ? 20 * Math.log10(SATURATION_REFERENCE / Math.SQRT2 / rms) : 48;
   return Math.max(-48, Math.min(48, gain));
 }