Cut a sound into named clips, as a texture into frames (#357)

A sound could only be cut to one stretch, set on the asset itself. To
play two parts of one file, the file had to be imported twice. Now a
sound is always the whole file, and its clips are the stretches of it,
each with a name and a cut of its own.

- A clip is { id, name, props } on SoundAsset.clips. Its id shares the
  generator with asset ids, so a sound field holds a sound or a clip,
  as a texture field holds a texture or a frame.
- props are the cut: start, end, the fades and their curves, reverse.
  Volume, loop, sound FX and bus stay on the sound for all its clips.
  The cut rows left the sound's schema for CLIP_ROWS.
- findSound resolves either id. Play sound, the Sample of an
  instrument, a Reverb impulse, song items, the scheduler and the test
  sound of the Sound FX tab all go through it.
- Every sound field offers each sound followed by its clips, "Kick",
  "Kick / Clip 1". A clip that is gone reads as "<id>?".
- The Sound tab has a strip under the ruler with the clips as bands.
  A drag there makes a clip, a click picks one, beside the bands the
  sound. Only the picked clip has handles. Reverse, fades and curves
  in the header work on it and are greyed out without one.
- Whole file is gone from the header, as the user asked.
- Picking a clip is a selection of its own, like a frame. The
  inspector shows its name, its cut rows and Delete clip. Delete
  takes it away without asking, as with a frame. Ctrl+Z brings it
  back.
- Copies get fresh clip ids: Duplicate, Embed a copy, a song
  duplicated with its takes, and the copy a split or paste makes of
  an embedded take. An item on a clip plays the same clip of its copy.
- A double-click on a song item playing a clip opens its sound with
  the clip picked. The Sound row of an item on an embedded take
  offers the take and its clips, which no list shows.
- A new take stays the whole sound. Clips are made by hand, as the
  user decided.
- No migration in the code, as the user decided. The one project
  with cut sounds was changed over by hand.

Deviation from the issue: deleting a used clip does not warn. A frame
does not warn either, and the issue had promised "like a frame".

Tests:
- Unit sound-clips (new): 11 green, 7 counter-checks each red
- Unit sounds, embedded: adjusted to clips, green
- e2e 34 (melektaus-atx): ALL PASSED, 90 checks (3 new, 12 rewritten)
- Counter-check in the browser: clips left out of the sound fields,
  the picker check fails
- Gate (melektaus-atx): CHECK EXIT 0, 271 files, 4194 tests

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

src/editor/sound/waveView.ts

@@ -147,10 +147,22 @@ export function draggedSpan(
   return b - a >= gap ? { start: a, end: b } : { start: a, end: Math.min(duration, a + gap) };
 }
 
-/**
- * What an asset stores for a span: seconds, an end at the end of the file as 0 – so a span over the
- * whole file is no cut at all, and plays the file itself.
- */
+/** A clip of the sound as its band in the strip (#357). */
+export interface ClipBand {
+  id: string;
+  span: CutSpan;
+}
+
+/** The clip whose band is at `x`, the one drawn last where bands lie over each other; null for none. */
+export function clipAt(view: WaveView, bands: readonly ClipBand[], x: number): string | null {
+  for (let i = bands.length - 1; i >= 0; i--) {
+    const { id, span } = bands[i]!;
+    if (x >= secondsToX(view, span.start) && x <= secondsToX(view, span.end)) return id;
+  }
+  return null;
+}
+
+/** What a clip stores for a span: seconds to the nearest sample, an end at the end of the file as 0. */
 export function storedCut(span: CutSpan, duration: number, rate: number): { start: number; end: number } {
   const sample = 1 / rate;
   const start = span.start < sample / 2 ? 0 : roundTo(span.start, rate);