Swirl and comb a sound with a Phaser and a Flanger node (#341)

sfxr's "phaser" is a comb filter, the sound plus a copy delayed by up
to 2.9 ms. That is a flanger, not the phaser a guitarist knows. Both
effects come in as nodes of their own, so each name fits its sound and
sfxr is covered in full. Both run on the FX worklet like the
Bitcrusher, because a feedback loop in Web Audio costs a render quantum
per round.

- Phaser: first-order allpasses (Stages 2, 4, 6, 8, 12) around
  Frequency, moved by a sine LFO Depth octaves wide at Rate Hz, with
  Feedback and Mix. An Envelope can move Frequency and Mix.
- Flanger: a delay line of Offset ms. Sweep moves it on in ms/s from
  the start of the voice, an LFO adds up to Depth ms. As in sfxr the
  delay is mirrored at 0, and it never goes past 20 ms. Feedback comes
  back exactly one delay later. An Envelope can move Offset and Mix.
- Mix 0 leaves the sound as it is, 1 is half sound and half copy. The
  notches are deepest there and the peaks stay at the sound's level.
  sfxr adds both at full, so its sound comes out 6 dB louder.
- The Flanger's processor gets the start of its voice and begins the
  sweep on that sample. stageNodes takes it as a new argument `at`.
- Explosion gets a Flanger in one draw out of two, with sfxr's offset
  and sweep and no LFO. Offset is offset^2 * 2.89 ms, Sweep is
  ramp^2 * 125 ms/s, signed so the direction holds.
- Randomize draws a Flanger's offset and sweep like sfxr. Its LFO and
  feedback and all of a Phaser are drawn within bounds of their own.
  Mix and Stages stay, as a Bitcrusher's Mix does. Mutate moves them
  too, and a 0 stays 0.
- New instrument preset Phased Pad: saw, Envelope, low-pass, Phaser of
  six stages. The other 14 are unchanged.
- The Bitcrusher's worklet setup is now shared with both nodes
  (workletNodes), its behaviour is unchanged.

Tests:
- Unit phaser (new): 8 green. Counter-check with clamping at 0 instead
  of mirroring and a flipped allpass coefficient: 3 red.
- Unit soundfx, sound-library, chip-presets: 45 / 76 / 14 green.
  Counter-checks: start time not passed on, phaser frequency eased
  linearly, sweep sign dropped, Phaser left out of Mutate, each red.
- Unit core, viewport, graph: 179 files, 2979 tests green.
- e2e 34 (melektaus-atx): ALL PASSED, 70 checks (1 new), first run.
  Chrome offline: 1 kHz notch RMS 0.0000 with two stages, 0.5657
  with four. Flanger echo after 48 samples, swept echo at 533/534.
  About 47 Phasers (12 stages) or 63 Flangers fill the audio thread.
- Gate (melektaus-atx): CHECK EXIT 0, 257 files, 3991 tests.

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

tools/e2e/34-sound.mjs

@@ -859,6 +859,86 @@ export async function sound(t) {
       crushed.halfOff < 1e-6,
     JSON.stringify(crushed),
   );
+  // #341: Phaser and Flanger on the worklet, rendered offline. A 1 kHz sine at 0.8 through a Phaser
+  //       at 1 kHz: two stages cut it out, four leave it whole. Swept 4 octaves wide once a second,
+  //       the notch passes it and is two octaves off between, where cos(28°) of it is left. A
+  //       Flanger of a voice starting at 0.25 s: an impulse 1 ms later comes back after 48 samples
+  //       at half. Sweeping 100 ms/s from that start, an impulse 0.1 s in comes back 533 samples
+  //       later. Then 64 stereo voices of each for 2 s, timed.
+  const swirled = await evaluate(`
+    (async () => {
+      const RATE = 48000;
+      const render = async (voices, seconds, channels, stage, fill, at = 0) => {
+        const context = new OfflineAudioContext(channels, RATE * seconds, RATE);
+        await context.audioWorklet.addModule(__gm.sound.fxWorkletUrl());
+        const sound = context.createBuffer(channels, RATE * seconds, RATE);
+        for (let c = 0; c < channels; c++) fill(sound.getChannelData(c));
+        for (let v = 0; v < voices; v++) {
+          const source = context.createBufferSource();
+          source.buffer = sound;
+          const nodes = __gm.sound.stageNodes(context, stage, at);
+          source.connect(nodes.input);
+          nodes.output.connect(context.destination);
+          source.start();
+        }
+        const started = performance.now();
+        const out = (await context.startRendering()).getChannelData(0);
+        return { out, ms: performance.now() - started };
+      };
+      const sine = (data) => {
+        for (let i = 0; i < data.length; i++) data[i] = Math.sin((2 * Math.PI * 1000 * i) / RATE) * 0.8;
+      };
+      const rms = (out, from, to) => {
+        let sum = 0;
+        for (let i = from; i < to; i++) sum += out[i] ** 2;
+        return Math.sqrt(sum / (to - from));
+      };
+      const phaser = (stages, rate, depth) =>
+        ({ kind: 'phaser', stages, frequency: 1000, rate, depth, feedback: 0, mix: 1 });
+      const notch = (await render(1, 0.3, 1, phaser(2, 0, 0), sine)).out;
+      const peak = (await render(1, 0.3, 1, phaser(4, 0, 0), sine)).out;
+      const swept = (await render(1, 1, 1, phaser(2, 1, 4), sine)).out;
+      const windows = Array.from({ length: 40 }, (_, w) => rms(swept, w * 1200, (w + 1) * 1200));
+      const flanger = (offset, sweep) =>
+        ({ kind: 'flanger', offset, sweep, rate: 0, depth: 0, feedback: 0, mix: 1 });
+      const impulse = (at) => (data) => { data[Math.round(at * RATE)] = 1; };
+      const heard = (out, from) => {
+        const found = [];
+        for (let i = from; i < from + 2000; i++) if (Math.abs(out[i]) > 1e-4) found.push([i - from, +out[i].toFixed(3)]);
+        return found;
+      };
+      const still = heard((await render(1, 0.5, 1, flanger(1, 0), impulse(0.251), 0.25)).out, Math.round(0.251 * RATE));
+      const sweeping = heard((await render(1, 0.5, 1, flanger(0, 100), impulse(0.35), 0.25)).out, Math.round(0.35 * RATE));
+      const noise = (data) => { for (let i = 0; i < data.length; i++) data[i] = Math.sin(i * 12.9898) % 1 * 0.5; };
+      const phaserLoad = await render(64, 2, 2, { ...phaser(12, 0.5, 2), feedback: 0.6 }, noise);
+      const flangerLoad = await render(64, 2, 2, { kind: 'flanger', offset: 1, sweep: 2, rate: 0.3, depth: 3, feedback: 0.5, mix: 1 }, noise);
+      return {
+        notch: +rms(notch, 4800, notch.length).toFixed(4),
+        peak: +rms(peak, 4800, peak.length).toFixed(4),
+        sweptLow: +Math.min(...windows).toFixed(3),
+        sweptHigh: +Math.max(...windows).toFixed(3),
+        still,
+        sweeping,
+        phaserVoices: Math.round((64 * 2000) / phaserLoad.ms),
+        flangerVoices: Math.round((64 * 2000) / flangerLoad.ms),
+      };
+    })()`);
+  check(
+    'the Phaser cuts a notch at its frequency and sweeps it, the Flanger comes back offset ms late and sweeps from its voice',
+    swirled.notch < 0.01 &&
+      Math.abs(swirled.peak - 0.8 * Math.SQRT1_2) < 0.01 &&
+      swirled.sweptLow < 0.1 &&
+      swirled.sweptHigh > 0.46 &&
+      swirled.sweptHigh < 0.51 &&
+      JSON.stringify(swirled.still) ===
+        JSON.stringify([
+          [0, 0.5],
+          [48, 0.5],
+        ]) &&
+      swirled.sweeping[0]?.[1] === 0.5 &&
+      JSON.stringify(swirled.sweeping.slice(1).map(([i]) => i)) === JSON.stringify([533, 534]),
+    JSON.stringify(swirled),
+  );
   // #329: the Chip on the worklet, rendered offline. A square at 220 Hz with a pitch of 12 and a duty
   //       of 0.25, starting at 0.25 s: silence until then, 440 periods a second after it, up a
   //       quarter of each. Sliding two octaves a second down from 440 Hz with a lowest frequency of
@@ -1215,7 +1295,7 @@ export async function sound(t) {
   const unled = await insGraph();
   check(
     'Preset → Square Lead plays every note at its own pitch and holds it; one undo takes it back',
-    ['Square Lead', 'Organ', 'Bell', 'Drum Kit'].every((label) => presetMenu.includes(label)) &&
+    ['Square Lead', 'Phased Pad', 'Organ', 'Bell', 'Drum Kit'].every((label) => presetMenu.includes(label)) &&
       leadGraph.chip?.followNote === true &&
       leadGraph.envelope?.sustain > 0 &&
       lead.last?.file === `instrument:${insId}` &&