Lay patterns and sounds out on tracks and play them as a song #334

Patterns could only be heard one at a time, in their own tab or from
a behavior. A song lays patterns and sounds out on tracks along one
time axis and plays them through the scheduler of #332, the third
piece of the music tools of #330.

- New asset type song under the heading Songs, with four empty tracks,
  120 BPM in 4/4 and a grid of a bar. Tempo, meter and grid are rows
  in the inspector. Tracks, items and the loop are the asset's data,
  held to their rules on load and in every command.
- A track has a name, volume, pan, mute and solo. Each is heard on a
  bus of its own, so a change reaches what is sounding at once.
- A pattern item repeats its pattern for as long as it is, with a
  transpose and another instrument if wanted. A sound item plays its
  sound as a clip at its own speed, round and round, through a sound
  FX if wanted. Both have a volume on top of their track's.
- Cut back at the left end or split, an item starts as far into what
  it plays as the cut is: ticks into a pattern, seconds into a sound.
- The tab shows the heads with M, S, volume and pan, the ruler of the
  Pattern tab and the items, a pattern with its notes, a sound with
  its waveform, a missing asset red. Patterns and sounds are dragged
  in from the Assets panel. Body, right and left end move, lengthen
  and cut back, Alt leaves the grid. Ctrl+C/V/D/A, Ctrl+E splits
  where playing starts, Delete, arrows. A double-click opens a
  pattern item in its tab.
- What is picked is a new selection of its own, so the inspector
  shows the item or the track, with a button to delete it.
- The scheduler plans clips beside notes. Where the playhead starts
  or jumps back inside a clip, it is taken up that far into its
  sound. A clip fades out at the end of its item or of the loop.
- The editor plays one piece at a time: a pattern started stops the
  song and the other way round.
- The rules are pure functions in core/music/song, editor/song/
  songView and songEdit.

Findings:
- The suites of 4ea752f and 2a3b90b were red at e2e 54: the Pattern
  pushed Struct below the fold of the + menu. The module now scrolls
  to it, as e2e 47 does for Vector art. It also left the menu open,
  which made e2e 55 fail behind it.
- The Assets panel only dragged assets a scene can place. Patterns
  and sounds are dragged now as well, and a scene says why it does
  not take one instead of calling every asset a scene.

Tests:
- song.test.ts (new, 14) and song-edit.test.ts (new, 9): model,
  round trip, sequence of patterns and clips, mute and solo, cuts,
  clips on the scheduler, moving, sizing, trimming, splitting,
  duplicating, hits, head parts. sound-library.test.ts +5: clip
  offset, end fade, pitched clip, track bus, peaks.
- e2e 65 (new) on melektaus-atx: ALL PASSED, 18 checks. e2e 54 and
  55 with the fix: ALL PASSED, 33 checks. e2e 4 with 30 asset
  types: ALL PASSED, 62 checks.
- Gate on melektaus-atx: CHECK EXIT 0, 250 test files, 3884 tests.
- Counter-checks: no taking up of a clip, no pattern offset, no clip
  end each fail 3, 1 and 1 tests.
- Probe on stein2: the tab with four tracks, before and while
  playing, looked at in two screenshots.

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

src/editor/pattern/PatternEditor.ts

@@ -22,6 +22,7 @@ import { Button, Dropdown, Label, Menu } from '../../ui';
 import { Music } from '../../viewport/music';
 import { soundLibrary } from '../../viewport/SoundLibrary';
 import { AssetTabEditor } from '../AssetTabEditor';
+import { claimTransport, releaseTransport } from '../music/transport';
 import { TOOLBAR_ICON_SVG } from '../panels/toolbarIcons';
 import type { EditorState } from '../state';
 import { PianoRoll } from './PianoRoll';
@@ -47,6 +48,7 @@ export class PatternEditor extends AssetTabEditor<PatternAsset, PianoRoll> {
   private playingId = '';
   /** What the scheduler was last told, so only a change is passed on. */
   private told = '';
+  private readonly halt = () => this.stop();
 
   constructor(state: EditorState) {
     super(state, { infoWidth: INFO_WIDTH });
@@ -89,6 +91,7 @@ export class PatternEditor extends AssetTabEditor<PatternAsset, PianoRoll> {
   play(): void {
     const asset = this.asset();
     if (!asset) return;
+    claimTransport(this.halt);
     this.playingId = asset.id;
     this.told = this.settingsOf(asset);
     void this.music.play(patternSequence(asset), {
@@ -101,6 +104,7 @@ export class PatternEditor extends AssetTabEditor<PatternAsset, PianoRoll> {
   }
 
   stop(): void {
+    releaseTransport(this.halt);
     this.music.stop();
     this.playingId = '';
     this.canvas.playhead = null;