Route sounds, songs and buses in one Master routing graph (#351)

A project bus could only go into one other bus, and a sound, an
instrument, a pattern or a song into exactly one bus. A shared reverb
needs a sound to go there in part and on dry at the same time. Instead
of a list of sends on every asset, the project gets one routing graph
like the routing of a song (#350), one level up.

- Menu -> Master routing opens a tab with one graph per project. It has
  a node per sound, instrument, pattern, song and bus, named after its
  asset, and one Master. Embedded assets have none, their song routes
  them.
- The nodes follow the assets. Every edit settles the routing in the
  same undo step (KeepRoutingCommand), so an undone delete brings the
  links of its asset back. A new asset gets a node and a link to the
  master. What went only into a deleted bus goes to the master.
- New nodes start in three columns: what plays, the buses, and the
  master a row above, so no link into it runs behind a bus.
- Links are sends: -60 to +12 dB, and a Mode on links out of a bus or a
  song. Out of a sound, an instrument or a pattern a link always takes
  the sound after its volume and FX. Loops are refused.
- A node without a link is silent and says "Goes nowhere".
- The rows Bus and Into are gone. Play nodes with a picked bus still
  overrule the routing.
- A click on a node shows its asset in the inspector, the Master shows
  the master volume and sound FX. A double click opens the asset's tab.
- The graph lives in project.routing and is held to the assets when a
  project is read, but nothing is added there. The editor adds the
  nodes when it opens a project. An asset without a node plays on the
  master, so old projects sound as before without a migration.
- In the mix an asset with one link at 0 dB goes straight into that bus.
  A sound or instrument with more gets a bus of its own with the sends.
  A pattern or song sends from the master of its playing, which is
  where its Mode taps. Buses of the mix can have sends now too.

Found:

- The check "the scheduler plans a looped bar" in e2e 34 wobbles. It
  failed in 2 of 5 runs here and in CI task 580 at 490fcba, before
  this change. HEAD passed it 2 of 2. Not fixed here.

Tests:

- Unit master-routing (new, 10), sound-library (2 new), buses
  rewritten onto the routing, 5 lists without the Bus row. 5 counter
  checks red.
- e2e 34 (melektaus-atx): ALL PASSED, 93 checks (3 new, 2 rewritten).
  Counter check with the Sound tab ignoring the routing: the routing
  check red.
- Gate (melektaus-atx): CHECK EXIT 0, 273 files, 4225 tests.

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

src/viewport/SoundLibrary.ts

@@ -35,7 +35,7 @@ import type { EaseShare } from '../core/model/animation';
 import { type BusLevel, Playbacks, type SequenceNeeds, type SequencePlay } from '../core/music/playbacks';
 import { arrivalOf, sendDelays, type TimedBus } from '../core/music/routing';
 import type { SendTap } from '../core/graph/Graph';
-import { MASTER_MIX, type MixBus, sameMix } from '../core/music/mixer';
+import { MASTER_MIX, type MixBus, type MixSend, sameMix } from '../core/music/mixer';
 import { limiterLatency } from '../core/soundfx/dynamics';
 import type { ScheduledClip, ScheduledNote } from '../core/music/Scheduler';
 import { dbToGain, SOUND_ITEM_MAX_DB } from '../core/music/song';
@@ -632,6 +632,7 @@ export class SoundLibrary implements AudioSink {
     this.cutPan(bus);
     bus.pan.connect(this.outOf(target));
     bus.into = target;
+    this.alignSends();
   }
 
   /**
@@ -714,9 +715,10 @@ export class SoundLibrary implements AudioSink {
   private async applyMix(): Promise<void> {
     const names = new Set(this.mix.map((bus) => bus.name));
     const waits: Promise<void>[] = [];
-    for (const { name, volume, into, fx } of this.mix) {
+    for (const { name, volume, into, sends, fx } of this.mix) {
       const bus = this.bus(name, into);
-      this.route(name, bus, into);
+      if (sends) this.setSends(bus, sends);
+      else this.route(name, bus, into);
       this.glideLevel(bus, this.mixVolumes.get(name) ?? volume);
       waits.push(this.setFx(bus, fx));
     }
@@ -943,7 +945,7 @@ export class SoundLibrary implements AudioSink {
     if (play.sequence.notes.length === 0 && !play.sequence.clips?.length) return 0;
     const id = this.nextId++;
     const buses = play.buses ?? [];
-    this.open(id, buses, play.fx ?? null, play.bus ?? MASTER_MIX);
+    this.open(id, buses, play.fx ?? null, play.bus ?? MASTER_MIX, play.sends);
     const tracks = new Set(buses.map((bus) => bus.name));
     const into = (bus: string | undefined) => groupBus(id, bus !== undefined && tracks.has(bus) ? bus : '');
     const { strike, strikeClip } = play;
@@ -992,14 +994,22 @@ export class SoundLibrary implements AudioSink {
 
   /**
    * The buses of a playing, its tracks into its master and a pattern item's into its track (#346),
-   * each with the sound FX it was given. Their chains are made once the worklet is there (`ready`).
+   * each with the sound FX it was given; its master into `into`, or along `sends` (#351). Their
+   * chains are made once the worklet is there (`ready`).
    */
-  private open(id: number, tracks: readonly BusLevel[], fx: FxVoice | null, into: string): void {
+  private open(
+    id: number,
+    tracks: readonly BusLevel[],
+    fx: FxVoice | null,
+    into: string,
+    sends?: readonly MixSend[],
+  ): void {
     const masterName = groupBus(id, '');
     const master = this.bus(masterName, into);
     master.gain.gain.value = 0;
     master.set = { gain: 0, pan: 0 };
     master.fx = fx;
+    if (sends) this.setSends(master, sends);
     const names = new Set(tracks.map((track) => track.name));
     const inside = (bus: string | undefined) =>
       bus !== undefined && names.has(bus) ? groupBus(id, bus) : masterName;