Play sounds, patterns and songs on the tracks of a composition (#418)

A trailer needs its music and effects on the same timeline as its
pictures. Until now a composition could only place objects, so sound
had to be started by hand from a behavior and drifted against the
picture.

- Audio items: a sound, a clip of a sound, a pattern or a song dragged
  from the asset panel onto a track becomes an item as long as one
  pass of it. It may share a track with object and video items and
  overlap them.
- Editing works as for other items: move, resize, trim on the left
  (moves the offset), fit to media, delete, copy and paste, box
  selection. Sounds draw their waveform, the colour tells the kind.
- The inspector shows what it plays, start, length, offset, volume
  (-60 to +12 dB) and bus. The bus is "From asset", the master or one
  of the buses, as asked for in the issue.
- An item longer than its material plays in rounds. Sounds and
  patterns start over, a song runs on in its loop. Loop marks on the
  item follow the same rule, a song's first mark sits at its loop end.
- compositionAudio.ts turns the items into sequences, each with its
  own tempo. CompositionSound plays them, in the tab and in a game.
- In the tab the items are heard from the playhead on, straight into
  the master like every tab. A scrub is silent. Playing a song or
  pattern in its own tab stops the composition, and the other way
  round.
- In a game they are planned on the audio clock for the moment the
  composition clock reaches them, with the scheduler's LEAD made up.
  While the clock stands (waiting for a video, paused, placement off)
  they are silent and resume at the right spot.

Found along the way:

- The asset panel never let a song out of its list, so a song could
  not be dragged anywhere. Song and scene tab reject it as they reject
  a pattern.
- The asset panel scrolled to the selected asset on every rebuild of
  its list, so folding a group far below the selection jumped back up.
  Only a new selection scrolls now.
- At the end of the list, folding a group pulled every row down by the
  rows that went away. ScrollContainer has a new opt-in holdOffset,
  set only in the asset panel, that keeps the offset until the user
  scrolls up.

Limits: a time scale other than 1 or a frame longer than 0.1 s moves
the picture and not the sound. A baked song plays its notes here, not
the bake.

Tests:
- tsc, eslint, Prettier clean. Gate on melektaus-atx green,
  309 files, 4788 tests (4772 plus 16 new).
- Unit: composition-audio.test.ts 13 new, scroll-container 3 new.
  Counter-checks: without catching up on load time or without the
  silence on pause the unit tests go red.
- e2e 68h: the sound is planned 1.2 ms after the composition reaches
  0.5 s. Without the LEAD make-up it is 54 ms late and red.
- e2e 04 and 69 ALL PASSED, 86 checks. 69o-ii drags a pattern and a
  song onto a track, 20 frames each.
- e2e 04: folding Sound at the end of the list keeps its header at
  y 234. With the old panel it jumps to 984 and the check is red.
- Not run in a browser: the loop marks of a song item, copy and paste
  of audio items, the bus choice.
- README and wiki count 2072 checks.

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

tests/ui/scroll-container.test.ts

@@ -67,6 +67,44 @@ describe('a scroll container with tail space', () => {
   });
 });
 
+describe('a scroll container holding its offset', () => {
+  const scrolledToEnd = (holdOffset: boolean) => {
+    const scroll = new ScrollContainer();
+    scroll.holdOffset = holdOffset;
+    scroll.resize(200, 100);
+    const rows = new VStack();
+    for (let i = 0; i < 10; i++) rows.add(new Spacer(20));
+    scroll.setContent(rows);
+    scroll.scrollTo(100);
+    // Three rows go, as a folded group's would.
+    rows.removeChildren(7);
+    scroll.relayoutContent();
+    return { scroll, rows };
+  };
+
+  it('keeps the offset when the content shrinks, the room left below', () => {
+    const { scroll, rows } = scrolledToEnd(true);
+    expect(scroll.offset).toBe(100);
+    expect(rows.h).toBe(200);
+  });
+
+  it('lets the room go as the view is scrolled up off it', () => {
+    const { scroll, rows } = scrolledToEnd(true);
+    scroll.scrollTo(70);
+    expect(rows.h).toBe(170);
+    expect(scroll.maxScroll).toBe(70);
+    scroll.scrollTo(20);
+    expect(rows.h).toBe(140);
+    expect(scroll.maxScroll).toBe(40);
+  });
+
+  it('without it clamps the offset, sliding every row down', () => {
+    const { scroll, rows } = scrolledToEnd(false);
+    expect(scroll.offset).toBe(40);
+    expect(rows.h).toBe(140);
+  });
+});
+
 describe('a scroll container and its scrollbar', () => {
   const filled = (viewportH: number, rowCount: number) => {
     const scroll = new ScrollContainer();