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/debug/songHooks.ts

@@ -0,0 +1,46 @@
+import { findAssetOf } from '../../core/model/Project';
+import type { HeadPart } from '../song/songView';
+import { soundLibrary } from '../../viewport/SoundLibrary';
+import { centreOf, type HookContext } from './hookHelpers';
+
+/** The Song tab and its tracks (#334). */
+export function songHooks({ state, editor }: HookContext) {
+  const { songEditor } = editor;
+  const timeline = songEditor.canvas;
+  return {
+    song: {
+      current: () => songEditor.currentAssetId,
+      info: () => songEditor.shownInfo,
+      rect: () => songEditor.canvasRect(),
+      /** The tracks, items and loop as the model holds them. */
+      model: (assetId: string) => {
+        const asset = findAssetOf(state.project, assetId, 'song');
+        return asset
+          ? (JSON.parse(
+              JSON.stringify({ tracks: asset.tracks, items: asset.items, loop: asset.loop ?? null }),
+            ) as unknown)
+          : null;
+      },
+      picked: () => timeline.pickedIds,
+      pickedTrack: () => timeline.pickedTrack,
+      /** Where a tick of a track's lane, a part of an item, a part of a head and a tick of the ruler are on screen. */
+      laneAt: (tick: number, track: number) => timeline.lanePoint(tick, track),
+      itemAt: (itemId: string, part?: 'start' | 'body' | 'end') => timeline.itemPoint(itemId, part),
+      headAt: (track: number, part: HeadPart, share?: number) => timeline.headPoint(track, part, share),
+      addTrackAt: () => timeline.addTrackPoint(),
+      rulerAt: (tick: number) => timeline.rulerPoint(tick),
+      view: () => timeline.viewInfo(),
+      items: () => timeline.itemsDrawn(),
+      playAt: () => ({ ...centreOf(songEditor.playButton), text: songEditor.playButton.text }),
+      fitAt: () => centreOf(songEditor.fitButton),
+      playing: () => songEditor.isPlaying,
+      /** The notes and clips the scheduler handed to the player since Play, and where it stands. */
+      planned: () => [...songEditor.music.planned],
+      clips: () => [...songEditor.music.clips],
+      position: () => songEditor.music.current?.position() ?? null,
+      clock: () => soundLibrary.clock(),
+      stats: () => soundLibrary.stats(),
+      buses: () => soundLibrary.busLevels(),
+    },
+  };
+}