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/runtime/systems/behaviorContext.ts

@@ -10,6 +10,7 @@ import type { Vec2 } from '../../core/model/Properties';
 import { JOINT_CONNECTED_KEY } from '../../core/model/joint';
 import { objectReference } from '../../core/model/objectRefs';
 import { findAsset, findAssetOf, findScene, type Project } from '../../core/model/Project';
+import { findSound } from '../../core/model/sound';
 import { renderLayerAlong } from '../../core/model/renderLayers';
 import { pickedFx } from '../../core/properties/assetFx';
 import { playedSoundById, playedSoundOf } from '../../core/properties/playedSound';
@@ -283,13 +284,13 @@ function playSound(
   tempo: number,
   bus: string,
 ): number {
-  const asset = findAssetOf(project, assetId, 'sound');
-  if (!asset?.file) return 0;
-  const props = resolveAssetProps(project, asset);
+  const ref = findSound(project, assetId);
+  if (!ref?.asset.file) return 0;
+  const props = resolveAssetProps(project, ref.asset);
   const level = Math.max(0, Math.min(1, volumeOf(props) * volume));
   const fx = fxVoiceOf(project, pickedFx(fxId, props), tempo, fxParams);
   return audio.play(
-    playedSoundOf(project, asset),
+    playedSoundOf(ref),
     level,
     loop ?? loopsOf(props),
     owner,
@@ -463,14 +464,14 @@ export function playClip(
   { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
   clip: ScheduledClip,
 ): number {
-  const asset = findAssetOf(project, clip.sound, 'sound');
-  if (!asset?.file) return 0;
+  const ref = findSound(project, clip.sound);
+  if (!ref?.asset.file) return 0;
   const volume = Math.max(
     0,
-    Math.min(dbToGain(SOUND_ITEM_MAX_DB), volumeOf(resolveAssetProps(project, asset)) * clip.volume),
+    Math.min(dbToGain(SOUND_ITEM_MAX_DB), volumeOf(resolveAssetProps(project, ref.asset)) * clip.volume),
   );
   return audio.playClip(
-    playedSoundOf(project, asset),
+    playedSoundOf(ref),
     { at: clip.at, offset: clip.offset, duration: clip.duration, volume, bus: clip.bus },
     clipFx(project, clip.fx, clip.tempo),
   );