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/panels/AssetPanel.ts

@@ -328,10 +328,14 @@ export class AssetPanel extends Panel {
     return this.list.rowColor(assetId);
   }
 
-  /** Hands the asset to the viewport under the pointer; a scene has nowhere to be placed into. */
+  /**
+   * Hands the asset to the view under the pointer: a scene places it, a song lays out a pattern or a
+   * sound (#334). A scene has nowhere to go.
+   */
   private startPlacementDrag(assetId: string): void {
     const asset = findAsset(this.state.project, assetId);
-    if (!asset || !isPlaceableType(asset.type)) return;
+    if (!asset || !(isPlaceableType(asset.type) || asset.type === 'pattern' || asset.type === 'sound'))
+      return;
     startDrag({ kind: 'asset', id: assetId }, asset.name);
   }
 

src/editor/panels/assetGroups.ts

@@ -38,6 +38,7 @@ export const ASSET_GROUP_LABELS: Record<AssetType, string> = {
   soundfx: 'Sound effects',
   instrument: 'Instruments',
   pattern: 'Patterns',
+  song: 'Songs',
   particles: 'Particles',
 };
 

src/editor/panels/assetIcons.ts

@@ -25,6 +25,7 @@ import rigidbody from './icons/rigidbody.svg?raw';
 import scene from './icons/scene.svg?raw';
 import shader from './icons/shader.svg?raw';
 import shaderfunction from './icons/shaderfunction.svg?raw';
+import song from './icons/song.svg?raw';
 import sound from './icons/sound.svg?raw';
 import soundfx from './icons/soundfx.svg?raw';
 import sprite from './icons/sprite.svg?raw';
@@ -57,6 +58,7 @@ export const ASSET_ICON_SVG: Record<AssetType, string> = {
   scene,
   shader,
   shaderfunction,
+  song,
   sound,
   soundfx,
   sprite,

src/editor/panels/icons/song.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-8 -8 16 16" width="16" height="16">
+  <!-- Tracks of a song: blocks laid out on lanes one above the other. -->
+  <path d="M-7 -2.2 H7 M-7 2.2 H7" fill="none" stroke="currentColor" stroke-width="0.9" stroke-opacity="0.6" />
+  <rect x="-6.5" y="-6" width="6" height="3" rx="0.6" fill="currentColor" />
+  <rect x="1" y="-6" width="5.5" height="3" rx="0.6" fill="currentColor" />
+  <rect x="-3.5" y="-1.5" width="7" height="3" rx="0.6" fill="currentColor" />
+  <rect x="-6.5" y="3" width="4" height="3" rx="0.6" fill="currentColor" />
+  <rect x="-1" y="3" width="7.5" height="3" rx="0.6" fill="currentColor" />
+</svg>