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/store/serialize.ts

@@ -39,6 +39,7 @@ import type {
   PixelFrame,
   PixelLayer,
   SceneAsset,
+  SoundClip,
   StateMachineAsset,
   TextureFrame,
   GradientStop,
@@ -112,6 +113,7 @@ import {
   variableDef,
 } from '../properties/PropertySchema';
 import { FIELD_SHADER_KEY, particleParamDefs } from '../properties/particles';
+import { CLIP_ROWS } from '../properties/sounds';
 import { SHADER_KEY, shaderParamDefs } from '../properties/shaders';
 
 /**
@@ -258,7 +260,8 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
       assets.push({ id, type, name, props, file });
     } else if (type === 'sound') {
       const file = typeof a.file === 'string' && isSoundFile(a.file) ? a.file : '';
-      assets.push({ id, type, name, props, file });
+      const clips = normalizeClips(a.clips, takenIds);
+      assets.push({ id, type, name, props, file, ...(clips.length > 0 && { clips }) });
     } else if (type === 'tile') {
       // A picture naming nothing is kept, like an animation's dangling root: it reads as missing
       // in the tab and comes back when the texture it named is restored by an undo.
@@ -738,6 +741,23 @@ function normalizeTexture(
   return { file, width, height, frames };
 }
 
+/** The clips of a sound (#357); like a frame's, a clip's id may not repeat one the document has. */
+function normalizeClips(raw: unknown, takenIds: Set<string>): SoundClip[] {
+  const clips: SoundClip[] = [];
+  for (const item of Array.isArray(raw) ? raw : []) {
+    const c = asRecord(item);
+    const id = typeof c.id === 'string' ? c.id : '';
+    if (!id || takenIds.has(id)) continue;
+    takenIds.add(id);
+    clips.push({
+      id,
+      name: typeof c.name === 'string' ? c.name : '',
+      props: normalizeBag(c.props, CLIP_ROWS, true),
+    });
+  }
+  return clips;
+}
+
 /**
  * The nine-slice and the sort line of a texture frame or a drawing (#308), measured against `size`.
  * A picture without them is one nobody sliced, and keeps saying so.