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/Editor.ts

@@ -75,6 +75,7 @@ import { ShaderEditor } from './shader/ShaderEditor';
 import { SoundFxEditor } from './soundfx/SoundFxEditor';
 import { InstrumentEditor } from './instrument/InstrumentEditor';
 import { PatternEditor } from './pattern/PatternEditor';
+import { SongEditor } from './song/SongEditor';
 import { StateMachineEditor } from './statemachine/StateMachineEditor';
 import { installShortcuts } from './shortcuts';
 import type { PlayFrom } from '../core/model/playSettings';
@@ -99,6 +100,7 @@ export type CentreTab =
   | 'soundfx'
   | 'instrument'
   | 'pattern'
+  | 'song'
   | 'game'
   | 'texture'
   | 'vectorart'
@@ -151,6 +153,7 @@ const PANE_ORDER = [
   'soundfx',
   'instrument',
   'pattern',
+  'song',
   'texture',
   'vectorart',
   'pixelart',
@@ -173,6 +176,7 @@ const TAB_LABELS: Record<CentreTab, string> = {
   soundfx: 'Sound FX',
   instrument: 'Instrument',
   pattern: 'Pattern',
+  song: 'Song',
   texture: 'Texture',
   vectorart: 'Vector art',
   pixelart: 'Pixel art',
@@ -224,6 +228,7 @@ export class Editor implements PlayHost {
   readonly soundFxEditor: SoundFxEditor;
   readonly instrumentEditor: InstrumentEditor;
   readonly patternEditor: PatternEditor;
+  readonly songEditor: SongEditor;
   readonly textureEditor: TextureEditor;
   readonly vectorArtEditor: VectorArtEditor;
   readonly pixelArtEditor: PixelArtEditor;
@@ -312,6 +317,7 @@ export class Editor implements PlayHost {
     this.soundFxEditor = new SoundFxEditor(state);
     this.instrumentEditor = new InstrumentEditor(state);
     this.patternEditor = new PatternEditor(state);
+    this.songEditor = new SongEditor(state);
     this.textureEditor = new TextureEditor(state);
     this.vectorArtEditor = new VectorArtEditor(state);
     this.pixelArtEditor = new PixelArtEditor(state);
@@ -423,6 +429,7 @@ export class Editor implements PlayHost {
       soundfx: this.soundFxEditor,
       instrument: this.instrumentEditor,
       pattern: this.patternEditor,
+      song: this.songEditor,
       texture: this.textureEditor,
       vectorart: this.vectorArtEditor,
       pixelart: this.pixelArtEditor,
@@ -495,6 +502,7 @@ export class Editor implements PlayHost {
         copy: () =>
           (this.tab === 'animation' && this.animationEditor.canvas.copyKeys()) ||
           (this.tab === 'pattern' && this.patternEditor.canvas.copy()) ||
+          (this.tab === 'song' && this.songEditor.canvas.copy()) ||
           copySelection(state),
         paste: () => this.paste(),
         removePoint: () => this.removePoint(),
@@ -514,7 +522,8 @@ export class Editor implements PlayHost {
     // creating it) is for – see the `open` event below.
     state.on('selection', () => {
       const s = state.selection;
-      if (s?.kind === 'node' || s?.kind === 'frame') this.openDocument(`asset:${s.assetId}`);
+      if (s?.kind === 'node' || s?.kind === 'frame' || s?.kind === 'song')
+        this.openDocument(`asset:${s.assetId}`);
       if (s?.kind === 'object' && findAsset(state.project, s.holderId)?.type === 'container')
         this.openDocument(`asset:${s.holderId}`);
     });
@@ -594,6 +603,8 @@ export class Editor implements PlayHost {
         return this.curveEditor.canvas.removePickedPoint();
       case 'pattern':
         return this.patternEditor.canvas.deleteSelection();
+      case 'song':
+        return this.songEditor.canvas.deleteSelection();
       default:
         return false;
     }
@@ -622,6 +633,8 @@ export class Editor implements PlayHost {
         return this.instrumentEditor.canvas.paste();
       case 'pattern':
         return this.patternEditor.canvas.paste();
+      case 'song':
+        return this.songEditor.canvas.paste();
       default:
         return false;
     }
@@ -1009,6 +1022,7 @@ export class Editor implements PlayHost {
     // The preview is the editor's own, so it runs on while the game is paused.
     if (this.tab === 'curve') this.curveEditor.tick(dt);
     if (this.tab === 'pattern') this.patternEditor.tick();
+    if (this.tab === 'song') this.songEditor.tick();
     // Play repaints the machine tab itself (see `PlaySession.tick`); an editor run has no session
     // to do it, and its states move just the same.
     if (this.tab === 'statemachine' && this.editorRun.isRunning) this.stateMachineEditor.canvas.redraw();
@@ -1042,7 +1056,8 @@ export class Editor implements PlayHost {
       (this.tab === 'animation' && this.animationEditor.isPlaying) ||
       (this.tab === 'tile' && this.tileEditor.isPlaying) ||
       (this.tab === 'curve' && this.curveEditor.isPlaying) ||
-      (this.tab === 'pattern' && this.patternEditor.isPlaying)
+      (this.tab === 'pattern' && this.patternEditor.isPlaying) ||
+      (this.tab === 'song' && this.songEditor.isPlaying)
     );
   }