Make a bus of the mix at its level, so a quiet bus starts quiet

A bus of the project's mix was made at unity and then glided to its
level. Where the audio context comes with the first sound of a run,
that sound already plays on the new bus. On a bus at volume 0 it was
heard for a moment, loud, while the level glided down.

- setMix now sets the level of a bus it makes at once, without a
  glide. A bus that is there already glides to a new level as before.
- Found while building #436: the e2e check with a sound on a bus at
  volume 0 measured -7.8 dB on that bus. With the fix it is silent.

Tests:
- New unit test in sound-library: buses made at their level with no
  glide, a later change glides. Red without the fix.
- sound-library 130/130, tsc clean on this change alone.

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

src/viewport/SoundLibrary.ts

@@ -909,10 +909,15 @@ export class SoundLibrary implements AudioSink {
     const names = new Set(this.mix.map((bus) => bus.name));
     const waits: Promise<void>[] = [];
     for (const { name, volume, into, sends, fx } of this.mix) {
+      const made = !this.buses.has(name);
       const bus = this.bus(name, into);
       if (sends) this.setSends(bus, sends);
       else this.route(name, bus, into);
-      this.glideLevel(bus, this.mixVolumes.get(name) ?? volume);
+      const level = this.mixVolumes.get(name) ?? volume;
+      // A bus made now is at its level at once: made with the context, it carries the first sound of
+      // a run, which would otherwise be heard through a glide down from unity (found by #436).
+      if (made) this.setLevel(bus, level);
+      else this.glideLevel(bus, level);
       waits.push(this.setFx(bus, fx));
     }
     const gone = [...this.buses.keys()].filter(
@@ -923,6 +928,13 @@ export class SoundLibrary implements AudioSink {
     await Promise.all(waits);
   }
 
+  /** The level of a bus at `gain` from now on, without a glide. */
+  private setLevel(bus: Bus, gain: number): void {
+    const to = Math.max(0, gain);
+    bus.gain.gain.value = to;
+    bus.set = { ...bus.set, gain: to };
+  }
+
   /** A quick glide of the level of a bus to `gain`. */
   private glideLevel(bus: Bus, gain: number, time = 0, ease: EaseShare = 'linear'): void {
     const to = Math.max(0, gain);

tests/viewport/sound-library.test.ts

@@ -2539,6 +2539,22 @@ describe('the master and the buses of the project (#325)', () => {
     expect(wires.filter((wire) => wire.endsWith('> destination'))).toEqual(['panner > destination']);
   });
 
+  it('makes a bus at its level, so the first sound of a run on a quiet bus is not heard gliding down (#436)', async () => {
+    const library = new SoundLibrary(false);
+    await library.setMix(mix);
+    await play(library);
+    expect(library.busLevels().map(({ name, level }) => [name, level])).toEqual([
+      ['mix', 0.5],
+      ['mix:a', 1],
+      ['mix:b', 0.25],
+    ]);
+    expect(gains.filter((gain) => gain.glides.length > 0)).toEqual([]);
+    // One there already glides to a new level, as before.
+    await library.setMix([bus('mix', 0.25, null), mix[1]!, mix[2]!]);
+    expect(library.busLevels()[0]).toMatchObject({ gain: 0.25, level: 0.5 });
+    expect(gains.some((gain) => gain.glides.includes(0.25))).toBe(true);
+  });
+
   it('keeps the mix where the first thing heard is a tab of a song, which names its bus before there is a context', async () => {
     const library = new SoundLibrary(false);
     await library.setMix(mix);