Duplicate an asset from its right-click menu (#297)

A new asset that starts out like an existing one (a death animation
next to "Break", a second slime) had to be built again by hand. Now
the asset panel's right-click menu has Duplicate.

- The copy lands right below the original and is named the way a new
  asset would be ("Crate 2"). It is selected and its tab opens where
  it has one. A copied scene does not become the active one.
- One undo step takes it out again. A redo brings back the same id.
- The copy is a deep copy: values, graph, tracks, layers and objects
  are its own, and editing it leaves the original alone.
- Frames of a texture or of pixel art get fresh ids, because a sprite
  names a frame across the whole document.
- Objects of a scene or container get fresh ids the way pasted ones
  do. Parents and object references among them follow the copy.
- Whatever pointed at the original keeps pointing at the original: a
  sprite wearing it, an animation's root, a placed object.

Tests:
- 3 new unit tests (DuplicateAssetCommand). One counter-check run:
  without renaming the references the scene test fails.
- e2e 04: 2 new checks with the real menu (copy named "Crate 2" right
  behind the Crate and selected, one undo removes it).
- Gate: 221 files, 3445 tests, CHECK EXIT 0.
- e2e total 1703 is computed (1701 + 2). Full suite left to CI.

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

README.md

@@ -45,7 +45,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1701 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1703 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs