Add the composition asset with its tracks, items and clips (#417)

A composition is a timeline of scene objects for cuts and cutscenes.
This first step brings the model and the pure evaluation behind it.
The tab and placing it in a scene follow in the next steps.

- CompositionAsset holds objects, tracks, items and clips. Each item
  owns exactly one object with overrides and container edits.
- The composition is a third object holder next to scene and
  container. Delete cascade, undo, usages and references work with it.
- Duplicating gives the copy new object ids and moves the item ids
  along with them.
- compositionAt() tells which items live at a frame. The top track is
  drawn in front.
- A clip acts on the item of its own track that it overlaps. After its
  end it holds the last value until the item ends. The animation's
  play mode is ignored, a clip is one pass.
- Clip targets are found by template path, else by name path, like
  the runtime's animation system.
- The root sprite of an item plays its video driven at offset plus
  time. Children follow unless they are already driven by hand.
- fittedLength() gives the video or clip length minus the offset, or
  3 s without media.
- fps (60) and loop live in their own properties file. Keeping them in
  PropertySchema would close an import cycle.
- The type is listed under Visuals with its own icon. It is not
  placeable yet.

Tests: the gate passed with 300 files and 4625 tests. That is 25 new
tests in tests/core/composition.test.ts and 2 icon checks that run
for every icon. Four counter-checks (track order, overlap rule, child
videos, ids on duplicate) each turned exactly their test red. e2e 04
passed with 67 checks and now counts 33 asset types.

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

tools/e2e/04-assets.mjs

@@ -2,7 +2,7 @@ import { colourDistance } from '../lib/harness.mjs';
 import { pickAssetType } from './lib/scaffold.mjs';
 
 /** One per entry of `ASSET_TYPES` – a new asset type counts here too. */
-const ASSET_TYPE_COUNT = 32;
+const ASSET_TYPE_COUNT = 33;
 /** The categories of the asset panel and the `+` menu, in their order (`ASSET_CATEGORIES`). */
 const CATEGORIES = ['Basics', 'Visuals', 'Logics', 'Physics', 'Audio'];