Keep both sounds full at the middle of a Mix with a curve #331

A Mix crossfaded: at 0.5 each sound was at half. Layering two sounds
in an instrument now goes through a Mix (076c564), so the user asked
for a second curve where each side stays full up to the middle and
only fades on the other half. At 0.25 a is full and b half, at 0.5
both are full, at 0.75 a is half and b full.

- The Mix has a setting Curve with Crossfade (default) and Both at
  center. A plan and a chain only say `curve` where it is `both`, so
  old projects and their compiled plans stay as they were.
- A share is not a straight line of `mix` with that curve. Two ramps
  of their own would dip to half each when `mix` glides across the
  middle. So a ConstantSourceNode carries `mix` on the audio thread,
  and two WaveShaperNodes make the shares from it. The knob of such a
  Mix is that one source.
- The source is started with the voice and stopped when it is cut or
  ends by itself, so it does not run on after the voice.
- A sound FX node in an instrument keeps the curve of a Mix inside it.
- At 0.5 both sounds are added in full, up to +6 dB where both are
  loud. That is what several edges into the Output did before. The
  wiki says so.

Found on the way: the suite of 23bdc3f (task 529) failed the Step
check in e2e 34. It timed the jump from the start of the voice, but a
timed change runs from when it arrives. In CI that was 80 ms later,
and the pitch rightly held to 2.08 s.

- stats() says `tunedAt`, the second of the audio clock the last
  change moved a voice. A change that moves nothing, as On update
  sends every frame, leaves it.
- The Step check times its readings from there.

Tests:
- sound-library.test.ts: both shares shaped from one source, a glide
  moves only the source, the source stops with a cut and with an end
  by itself (50 tests, 2 new)
- soundfx.test.ts: curve only where set, one sound at 0 (41 tests)
- instrument.test.ts: the curve through a sound FX node (34 tests)
- Counter-checks: dropping the curve in throughFx and not stopping
  the source in done() each turn their new test red
- e2e 34 on melektaus-atx: ALL PASSED, 48 checks (+1). Rendered
  offline in Chrome, a and b come to 1/0, 1/0.5, 1/1, 0.5/1 and 0/1
  at mix 0 to 1, and a glide from 0 to 1 reads the same at 0.25, 0.5
  and 0.75 s. Step jumped 2.04 s after the change.
- Gate on melektaus-atx: CHECK EXIT 0, 244 test files, 3793 tests

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

tools/e2e/34-sound.mjs

@@ -578,8 +578,9 @@ export async function sound(t) {
   await play();
   const pitchNow = () =>
     json(`({ pitch: __gm.sound.levels(${JSON.stringify(blip.file)})?.[0]?.[0], clock: __gm.sound.clock() })`);
-  const sinceStart = async (seen) => {
-    const from = (await stats()).last.at;
+  const sinceStart = async (seen, since = 'start') => {
+    const now = await stats();
+    const from = since === 'tune' ? now.tunedAt : now.last.at;
     return seen
       .filter((s) => typeof s.pitch === 'number')
       .map((s) => ({ t: +(s.clock - from).toFixed(3), pitch: +s.pitch.toFixed(3) }));
@@ -596,12 +597,16 @@ export async function sound(t) {
   );
   await stop();
   // Step holds the pitch for the whole time and then jumps there: every reading before 2 s on the
-  // audio clock is still 7, the first one past it is the end.
+  // audio clock is still 7, the first one past it is the end. Timed from the change, not from the
+  // start of the voice: task 529 saw it arrive 80 ms later and the 7 held, rightly, to 2.08 s.
   await mutate(`
     const tune = __gm.state.project.assets.find((a) => a.id === 'sn-logic').graph.nodes.find((n) => n.id === 'sn-tune');
     Object.assign(tune.params, { ease: 'step', time: 2 });`);
   await play();
-  const steps = await sinceStart(await trace(pitchNow, (s) => typeof s.pitch === 'number' && s.pitch < 6));
+  const steps = await sinceStart(
+    await trace(pitchNow, (s) => typeof s.pitch === 'number' && s.pitch < 6),
+    'tune',
+  );
   const held = steps.filter(({ pitch }) => pitch === 7);
   const jumped = steps.find(({ pitch }) => pitch < 6);
   check(
@@ -750,6 +755,50 @@ export async function sound(t) {
       [echo.late, echo.latePlain].every((late) => late.every((n, k) => n === 0 || n === k * 128)),
     JSON.stringify(echo),
   );
+  // A Mix "Both at center", rendered offline: a steady 1 on a and 0.5 on b in two channels, so each
+  // share reads off as it is. Then mix glides 0 → 1 over a second: at its middle both stay full.
+  const atCenter = await evaluate(`
+    (async () => {
+      const RATE = 48000;
+      const render = async (mix, glide) => {
+        const context = new OfflineAudioContext(2, RATE, RATE);
+        const merger = context.createChannelMerger(2);
+        merger.connect(context.destination);
+        const shareA = context.createGain();
+        const shareB = context.createGain();
+        shareA.gain.value = 0;
+        shareB.gain.value = 0;
+        const driver = __gm.sound.bothAtCenter(context, mix, shareA, shareB);
+        for (const [share, level, side] of [[shareA, 1, 0], [shareB, 0.5, 1]]) {
+          const steady = context.createConstantSource();
+          steady.offset.value = level;
+          steady.connect(share).connect(merger, 0, side);
+          steady.start();
+        }
+        if (glide) driver.offset.setValueAtTime(0, 0).linearRampToValueAtTime(1, 1);
+        driver.start();
+        const out = await context.startRendering();
+        const at = (s) => [out.getChannelData(0)[Math.round(s * RATE)], out.getChannelData(1)[Math.round(s * RATE)] * 2];
+        return glide ? [0.25, 0.5, 0.75].map(at) : at(0.5);
+      };
+      const round = (pair) => pair.map((v) => Math.round(v * 1000) / 1000);
+      const fixed = {};
+      for (const mix of [0, 0.25, 0.5, 0.75, 1]) fixed[mix] = round(await render(mix, false));
+      return { fixed, glide: (await render(0, true)).map(round) };
+    })()`);
+  const pairNear = (pair, a, b) => near(pair[0], a) && near(pair[1], b);
+  check(
+    'a Mix "Both at center" keeps a full up to the middle and b full from it, also while mix glides',
+    pairNear(atCenter.fixed[0], 1, 0) &&
+      pairNear(atCenter.fixed[0.25], 1, 0.5) &&
+      pairNear(atCenter.fixed[0.5], 1, 1) &&
+      pairNear(atCenter.fixed[0.75], 0.5, 1) &&
+      pairNear(atCenter.fixed[1], 0, 1) &&
+      pairNear(atCenter.glide[0], 1, 0.5) &&
+      pairNear(atCenter.glide[1], 1, 1) &&
+      pairNear(atCenter.glide[2], 0.5, 1),
+    JSON.stringify(atCenter),
+  );
   await mutate(`
     const logic = __gm.state.project.assets.find((a) => a.id === 'sn-logic');
     logic.variables = [];