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/core/instrument/compile.ts

@@ -15,6 +15,7 @@ import type { InstrumentAsset } from '../model/Asset';
 import { graphLookup } from '../model/graphLookup';
 import { textOf } from '../model/Properties';
 import { findAssetOf, type Project } from '../model/Project';
+import { findSound } from '../model/sound';
 import { playedSoundOf } from '../properties/playedSound';
 import { resolveAssetProps } from '../properties/resolve';
 import { volumeOf } from '../properties/sounds';
@@ -77,19 +78,18 @@ export function instrumentSourcesOf(project: Project): InstrumentSources {
   return {
     lookup: graphLookup(project),
     sound: (assetId) => {
-      const asset = findAssetOf(project, assetId, 'sound');
-      if (!asset) return null;
+      const ref = findSound(project, assetId);
+      if (!ref) return null;
       // An asset without a file stays without one, so the Sample can say so.
-      const file = asset.file ? playedSoundOf(project, asset) : '';
-      return { file, volume: volumeOf(resolveAssetProps(project, asset)) };
+      return { file: playedSoundOf(ref), volume: volumeOf(resolveAssetProps(project, ref.asset)) };
     },
     soundFx: (assetId) => {
       const asset = findAssetOf(project, assetId, 'soundfx');
       return asset ? soundFxBuildFor(asset, project).compiled : null;
     },
     key: (assetId) => {
-      const sound = findAssetOf(project, assetId, 'sound');
-      if (sound) return `${playedSoundOf(project, sound)}@${volumeOf(resolveAssetProps(project, sound))}`;
+      const sound = findSound(project, assetId);
+      if (sound) return `${playedSoundOf(sound)}@${volumeOf(resolveAssetProps(project, sound.asset))}`;
       const fx = findAssetOf(project, assetId, 'soundfx');
       return fx ? soundFxKey(fx, project) : '';
     },