Mix sounds through buses and a master (#325)

Until now every sound went straight to the speakers. There was no
point where all of them came together, so nothing could turn the
music down against the effects, and a compressor or a shared echo
over everything had nowhere to sit. The bus technique of #346 was
there already, only the end of the chain was missing.

- Bus asset (Audio): Volume, Sound FX and Into. Into is empty for
  the master and never offers the bus itself nor a bus that already
  goes into it. The list is worked out when the menu opens, so a
  change made meanwhile counts. A loop that is in a project anyway
  sends the buses on it into the master.
- The master is no asset. Its volume and sound FX are two new rows
  under Master in Project settings, one undo step each. A project
  written before has 1 and none, which is how it sounded.
- Sound, instrument, pattern and song have a row Bus, empty for the
  master. Play sound, Play note, Play pattern and Play song have an
  input bus: From the asset, Master or a bus. Inside a song or a
  pattern only the bus of the song or pattern counts.
- Set bus volume (bus, volume, time, easing) and Bus volume. An
  empty bus is the master. The volume set holds for the whole run,
  across scenes, over that of the asset. GameRun resets it at start
  and end. Set sound FX parameters without a voice now reaches the
  sound FX of the bus picked in its new bus input.
- SoundLibrary: the master is the bus "mix", each bus "mix:<id>".
  Every bus, the playing masters of #346 included, ends there, and
  only the master is heard. setMix sets them up once a context
  exists, so an editor nobody listened to opens no audio device. The
  same mix again changes nothing, which lets the editor hand it over
  on every change of the project.
- The tabs play as the game does: Sound, Sound FX, Instrument,
  Pattern and Song tab through the bus of their asset and the master.
- Pause game does nothing to the buses (the user's answer 4b). A
  game that wants quiet in its pause menu sets a bus volume itself.
- Streaming long music files is #349, split off as asked.

Finding on the way: the Into menu first worked from a list made when
the inspector row was built. After another bus was sent into the
open one, e2e 34 saw it still offered. It is now asked on open.

The SoundLibrary tests expected voices to end at the destination.
They now end in the master, which adds its input, level and pan to
every wiring and two gains to every list. Those expectations are
updated, nothing else in them changed.

Tests:
- Unit buses (new): 15 green. Ten counter-checks, each red on its
  own: loop check, bus picked, reset at run start, bus of Set sound
  FX parameters, master volume in the settings, and in SoundLibrary
  default into the master, voices into the master, run volume over
  the mix, no context before a play, a playing on its bus.
- Unit sound-library: 90 green (5 new). Node catalog: 416 green
  (9 new cases).
- e2e 34 (melektaus-atx): ALL PASSED, 75 checks (3 new). The voice
  of the Sound tab was heard on mix:<bus>, that bus into the other,
  that one at 0.5 into the master. Master volume 0.6 via the dialog,
  undo back to 1.
- Gate (melektaus-atx): CHECK EXIT 0, 262 files, 4072 tests.

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

tools/e2e/34-sound.mjs

@@ -1724,6 +1724,86 @@ export async function sound(t) {
   );
   await shot('34-sound-tab');
 
+  // 34h-3. Buses and the master (#325). "+ → Bus" makes one into the master; its Into never offers
+  //        itself nor a bus that goes into it; the Sound tab plays a sound on its bus, and the
+  //        master's volume is a project setting, one undo step.
+  for (let i = 0; i < 2; i++) {
+    await clickText('+');
+    await pickAssetType(t, 'Bus');
+    await sleep(150);
+  }
+  const [busA, busB] = await assetsOfType('bus');
+  const intoMenu = async () => {
+    await select({ kind: 'asset', assetId: busA.id });
+    const at = await json("__gm.inspectorField('Into', true)");
+    await click(at.x, at.y);
+    const offered = await json('__gm.overlayTexts()');
+    await press('Escape');
+    return offered;
+  };
+  const offeredBefore = await intoMenu();
+  await mutate(`
+    const p = __gm.state.project;
+    p.assets.find((a) => a.id === ${JSON.stringify(busB.id)}).props.into = ${JSON.stringify(busA.id)};`);
+  const offeredAfter = await intoMenu();
+  check(
+    '"+ → Bus" makes buses into the master, and Into offers neither the bus itself nor one going into it',
+    [busA, busB].every((bus) => bus.props.volume === 1 && bus.props.fx === '' && bus.props.into === '') &&
+      JSON.stringify(offeredBefore) === JSON.stringify(['Search…', 'Master', `${busB.name} (bus)`]) &&
+      JSON.stringify(offeredAfter) === JSON.stringify(['Search…', 'Master']),
+    JSON.stringify({ busA, busB, offeredBefore, offeredAfter }),
+  );
+
+  await mutate(`
+    const p = __gm.state.project;
+    p.assets.find((a) => a.id === ${JSON.stringify(busA.id)}).props.volume = 0.5;
+    p.assets.find((a) => a.id === ${JSON.stringify(droneId)}).props.bus = ${JSON.stringify(busB.id)};`);
+  await evaluate(`__gm.state.openAsset(${JSON.stringify(droneId)})`);
+  const startedBefore = (await stats()).started;
+  await click(tabPlay.x, tabPlay.y);
+  await waitFor(stats, (s) => s.started > startedBefore, 10);
+  const buses = await json('__gm.sound.buses()');
+  const heardOn = (await stats()).last?.bus;
+  const levelOf = (name) => buses.find((bus) => bus.name === name);
+  check(
+    'the Sound tab plays the sound on its bus, into the bus that one names, on into the master',
+    heardOn === `mix:${busB.id}` &&
+      levelOf(`mix:${busB.id}`)?.into === `mix:${busA.id}` &&
+      levelOf(`mix:${busA.id}`)?.into === 'mix' &&
+      levelOf(`mix:${busA.id}`)?.gain === 0.5 &&
+      levelOf('mix')?.into === null,
+    JSON.stringify({ heardOn, buses }),
+  );
+
+  await t.clickToolbar('menu');
+  await clickText('Project settings…');
+  const masterAt = await json('__gm.projectSettingsMasterAt()');
+  await click(masterAt.volume.x, masterAt.volume.y);
+  await press('a', { ctrl: true });
+  await t.typeText('0.6');
+  await press('Enter');
+  await clickText('Apply');
+  await sleep(200);
+  const mastered = {
+    project: await json('__gm.state.project.masterVolume'),
+    heard: await json("__gm.sound.mixVolume('mix')"),
+  };
+  await t.clickToolbar('undo');
+  await sleep(200);
+  const undone = {
+    project: await json('__gm.state.project.masterVolume'),
+    heard: await json("__gm.sound.mixVolume('mix')"),
+  };
+  check(
+    'the master volume is a project setting the mix follows, and undo gives the old one back',
+    mastered.project === 0.6 && mastered.heard === 0.6 && undone.project === 1 && undone.heard === 1,
+    JSON.stringify({ masterAt, mastered, undone }),
+  );
+  await mutate(`
+    const p = __gm.state.project;
+    p.assets.find((a) => a.id === ${JSON.stringify(droneId)}).props.bus = '';
+    p.assets = p.assets.filter((a) => a.type !== 'bus');`);
+
   // 34i. The step takes back everything it made, so the steps after it see the fixture again.
   await mutate(`
     const gone = new Set(['sn-logic', ...${JSON.stringify(both.map((a) => a.id))}]);