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/editor/routing/RoutingEditor.ts

@@ -0,0 +1,63 @@
+import { masterRoutingProblems } from '../../core/music/projectRouting';
+import { ROUTING_ID } from '../../core/model/Project';
+import { Button, Label, Spacer, VStack } from '../../ui';
+import { GraphCanvas } from '../graph/GraphCanvas';
+import { SnapToggle } from '../graph/SnapToggle';
+import type { EditorState } from '../state';
+import { TabHeader } from '../TabHeader';
+
+const INFO_WIDTH = 220;
+
+/**
+ * The Master routing tab (#351), one per project: a node per sound, instrument, pattern, song and bus
+ * and the master, the links between them where each one goes. Edited as the routing of a song is
+ * (#350); a click on a node shows its asset in the inspector, a double click opens it.
+ */
+export class RoutingEditor extends VStack {
+  readonly canvas: GraphCanvas;
+  readonly fitButton: Button;
+  readonly snap: SnapToggle;
+  private readonly info = new Label('', { style: 'small', align: 'right' });
+
+  constructor(private readonly state: EditorState) {
+    super();
+    this.canvas = new GraphCanvas(state, 'routing');
+    this.snap = new SnapToggle(() => this.canvas);
+    this.fitButton = new Button('Fit', { onClick: () => this.canvas.fit() });
+    this.info.minWidth = INFO_WIDTH;
+    const header = new TabHeader();
+    for (const widget of this.snap.widgets) header.add(widget);
+    header.add(this.fitButton);
+    header.add(new Spacer(), 1);
+    header.add(this.info);
+    this.add(header);
+    this.add(this.canvas, 1);
+    state.on('project', () => this.refresh());
+    state.on('load', () => this.close());
+  }
+
+  /** Shows the routing of the project; framed the first time, as it was left after that. */
+  open(): void {
+    this.canvas.show(ROUTING_ID);
+    this.refresh();
+  }
+
+  /** The tab is closed: opened again, the routing starts from a fitted view. */
+  close(): void {
+    this.canvas.show('');
+    this.refresh();
+  }
+
+  /** The readout: how many links, and how many nodes go nowhere and are never heard. */
+  get shownInfo(): string {
+    return this.info.text;
+  }
+
+  private refresh(): void {
+    if (this.canvas.currentAssetId !== ROUTING_ID) return this.info.setText('');
+    const graph = this.state.project.routing;
+    const silent = Object.keys(masterRoutingProblems(graph)).length;
+    const links = `${graph.edges.length} link${graph.edges.length === 1 ? '' : 's'}`;
+    this.info.setText(silent > 0 ? `${links} · ${silent} going nowhere` : links);
+  }
+}