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/core/model/Asset.ts

@@ -1,5 +1,6 @@
 import type { Graph } from '../graph/Graph';
 import type { PatternNote } from '../music/pattern';
+import type { SongItem, SongTrack } from '../music/song';
 import type { LoopRange } from '../music/sequence';
 import type { PropertyDef } from '../properties/PropertySchema';
 import type { AssetType } from './AssetTypes';
@@ -341,6 +342,18 @@ export interface PatternAsset extends AssetBase {
   loop?: LoopRange;
 }
 
+/**
+ * Sounds and patterns on tracks along one time axis (#334). The tempo, the meter and the grid are
+ * properties; the tracks, their items and the loop are the asset's own data, the whole song while
+ * no loop is set.
+ */
+export interface SongAsset extends AssetBase {
+  type: 'song';
+  tracks: SongTrack[];
+  items: SongItem[];
+  loop?: LoopRange;
+}
+
 /**
  * A tile a tilemap paints with: `pictures` names one block per animation step (a texture id for a
  * sheet that holds nothing else, a frame id for one block cut out of a sheet), and `props` say
@@ -596,6 +609,7 @@ export type Asset =
   | SoundFxAsset
   | InstrumentAsset
   | PatternAsset
+  | SongAsset
   | ParticlesAsset;
 
 /** The assets whose `variables` are declared by the user – see `requireVariableOwner`. */

src/core/model/AssetTypes.ts

@@ -27,6 +27,7 @@ export const ASSET_TYPES = [
   'soundfx',
   'instrument',
   'pattern',
+  'song',
   'particles',
 ] as const;
 
@@ -61,6 +62,7 @@ export const ASSET_TYPE_LABELS: Record<AssetType, string> = {
   soundfx: 'Sound FX',
   instrument: 'Instrument',
   pattern: 'Pattern',
+  song: 'Song',
   particles: 'Particles',
 };
 
@@ -74,7 +76,7 @@ export const ASSET_TYPE_LABELS: Record<AssetType, string> = {
  * A struct is a shape that variables and fields take on (#253), nothing that exists on its own.
  * A shader function is part of other shaders (#303), worn by nothing itself. A sound FX is what a
  * sound is played through (#311), heard but never seen, an instrument what a note is played
- * with (#331) and a pattern the notes it plays (#333).
+ * with (#331), a pattern the notes it plays (#333) and a song what lays them out (#334).
  */
 export type PlaceableAssetType = Exclude<
   AssetType,
@@ -94,6 +96,7 @@ export type PlaceableAssetType = Exclude<
   | 'soundfx'
   | 'instrument'
   | 'pattern'
+  | 'song'
   | 'variables'
   | 'struct'
 >;
@@ -115,6 +118,7 @@ const NOT_PLACEABLE: ReadonlySet<AssetType> = new Set([
   'soundfx',
   'instrument',
   'pattern',
+  'song',
   'variables',
   'struct',
 ]);

src/core/model/placement.ts

@@ -1,5 +1,5 @@
 import type { Asset } from './Asset';
-import { isPlaceableType } from './AssetTypes';
+import { ASSET_TYPE_LABELS, isPlaceableType } from './AssetTypes';
 import { containerReaches } from './container';
 import type { ObjectHolder, Project } from './Project';
 
@@ -9,7 +9,9 @@ import type { ObjectHolder, Project } from './Project';
  * promise a placement the command then refuses.
  */
 export function placementRejection(project: Project, holder: ObjectHolder, asset: Asset): string {
-  if (!isPlaceableType(asset.type)) return 'A scene cannot be placed into a scene';
+  // A pattern or a sound is dragged out of the panel for a song (#334), and may pass over a scene.
+  if (!isPlaceableType(asset.type))
+    return `A ${ASSET_TYPE_LABELS[asset.type].toLowerCase()} cannot be placed into a scene`;
   if (asset.type === 'container' && containerReaches(project, asset, holder.id))
     return `${asset.name} cannot contain ${holder.name}: that nests forever`;
   return '';