Shape a sound with every filter type, a Q and a slope (#318)

The Filter node from #335 only knew low-pass, high-pass and band-pass
and had a Resonance from 0 to 1. It now has every type Web Audio
offers, a Q as a plain factor and a slope up to 48 dB per octave.

- Types: low-pass, high-pass, band-pass, notch, low shelf, high shelf,
  peaking and all-pass. The cutoff row is now labelled Frequency, its
  key stays `cutoff`.
- Q from 0.1 to 30, 0.707 by default, replaces Resonance. Web Audio
  takes the Q of a low- or high-pass in dB, so it is converted when the
  voice is wired. No project used the node.
- Gain in dB is an input only for the shelves and the peak. It is a
  derived port, so the sound FX compiler now resolves derived ports.
- Slope 12, 24 or 48 dB/oct runs 1, 2 or 4 filters in a row. They
  share one cutoff carried by a ConstantSource, only the last has the
  Q. Shelves, peak and all-pass hide the row.
- The Chip Presets get the Q their resonance gave before. Mutate moves
  Q in octaves and gain by up to a dB.
- bindStage in the instrument compiler binds every amount of a stage
  generically, so new stages need no case of their own there.

Finding: FLAT_Q was in dB and the delay damping used it. It is a
factor now, the damping takes it through passQ and is unchanged.

Tests:
- Unit envelope 13, sound-library 96 (2 new), chip-presets 14,
  soundfx 45, all green.
- Counter-checks: the Q on every filter in a row, the gain row for
  every type. Each turns one test red.
- e2e 34 (melektaus-atx): ALL PASSED, 76 checks (1 new). Chrome
  offline, 2 kHz through a low-pass at 1 kHz: -12.37 dB at 12 dB/oct,
  -49.5 dB at 48. A +6 dB peak gives 6.00 dB, a notch -124 dB.
- The gate runs once on the last commit of this series.

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

src/viewport/SoundLibrary.ts

@@ -2,6 +2,7 @@ import {
   DELAY_MAX,
   DRY_VOICE,
   FLAT_Q,
+  passQ,
   fxChainOf,
   type FxChain,
   type FxChip,
@@ -2048,22 +2049,48 @@ export function stageNodes(
   };
 }
 
-/** A Filter (#335): a cutoff that moves in octaves, and a Q – a band-pass's as a factor, so in ratios too. */
+/**
+ * A Filter (#335, #318): a cutoff that moves in octaves, a Q – as a factor in ratios too, in dB for a
+ * low- or high-pass – and a gain in dB. Several in a row share one cutoff, carried by a source of its
+ * own into each of them; only the last has the Q, the others none.
+ */
 function passNodes(context: BaseAudioContext, stage: FxPass, follow?: Follow): StageNodes {
-  const filter = context.createBiquadFilter();
-  filter.type = stage.filter;
-  filter.frequency.value = stage.cutoffMod ? 0 : stage.cutoff;
-  filter.Q.value = stage.qMod ? 0 : stage.q;
-  const qEven = stage.filter === 'bandpass' ? 'ratio' : 'value';
-  if (stage.cutoffMod) follow?.(stage.cutoffMod, filter.frequency, true);
-  if (stage.qMod) follow?.(stage.qMod, filter.Q, qEven === 'ratio');
+  const qEven = stage.filter === 'lowpass' || stage.filter === 'highpass' ? 'value' : 'ratio';
+  const filters = Array.from({ length: Math.max(1, stage.stages) }, () => {
+    const filter = context.createBiquadFilter();
+    filter.type = stage.filter;
+    filter.Q.value = passQ(stage.filter, FLAT_Q);
+    return filter;
+  });
+  filters.slice(1).forEach((filter, i) => filters[i]!.connect(filter));
+  const last = filters.at(-1)!;
+  last.Q.value = stage.qMod ? 0 : stage.q;
+  last.gain.value = stage.gainMod ? 0 : stage.gain;
+  if (stage.qMod) follow?.(stage.qMod, last.Q, qEven === 'ratio');
+  if (stage.gainMod) follow?.(stage.gainMod, last.gain, false);
+  let cutoff = filters[0]!.frequency;
+  let stop: (() => void) | undefined;
+  if (filters.length > 1) {
+    const driver = context.createConstantSource();
+    for (const filter of filters) {
+      filter.frequency.value = 0;
+      driver.connect(filter.frequency);
+    }
+    driver.start();
+    cutoff = driver.offset;
+    stop = () => driver.stop();
+  }
+  cutoff.value = stage.cutoffMod ? 0 : stage.cutoff;
+  if (stage.cutoffMod) follow?.(stage.cutoffMod, cutoff, true);
   return {
-    input: filter,
-    output: filter,
+    input: filters[0]!,
+    output: last,
     knobs: [
-      { param: stage.cutoffMod ? null : filter.frequency, target: stage.cutoff, even: 'ratio' },
-      { param: stage.qMod ? null : filter.Q, target: stage.q, even: qEven },
+      { param: stage.cutoffMod ? null : cutoff, target: stage.cutoff, even: 'ratio' },
+      { param: stage.qMod ? null : last.Q, target: stage.q, even: qEven },
+      { param: stage.gainMod ? null : last.gain, target: stage.gain, even: 'value' },
     ],
+    ...(stop && { stop }),
   };
 }
 
@@ -2129,7 +2156,7 @@ function delayNodes(context: BaseAudioContext, stage: FxDelay): StageNodes {
   const damp = context.createBiquadFilter();
   damp.type = 'lowpass';
   damp.frequency.value = stage.damping;
-  damp.Q.value = FLAT_Q;
+  damp.Q.value = passQ('lowpass', FLAT_Q);
   const feedback = gainNode(context, stage.feedback);
   const wet = gainNode(context, stage.mix);
   line.connect(damp);
@@ -2384,7 +2411,7 @@ function levelsOf(chain: FxChain, into: number[] = []): number[] {
   }
   for (const stage of chain.stages)
     if (stage.kind === 'delay') into.push(stage.time, stage.feedback, stage.damping, stage.mix);
-    else if (stage.kind === 'pass') into.push(stage.cutoff, stage.q);
+    else if (stage.kind === 'pass') into.push(stage.cutoff, stage.q, stage.gain);
     else if (stage.kind === 'crush') into.push(stage.bits, stage.rate, stage.mix);
     else if (stage.kind === 'phaser')
       into.push(stage.frequency, stage.rate, stage.depth, stage.feedback, stage.mix);