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>

src/editor/panels/AssetPanel.ts

@@ -5,6 +5,7 @@ import {
 } from '../../core/commands/animationCommands';
 import {
   CreateAssetCommand,
+  DuplicateAssetCommand,
   SetActiveSceneCommand,
   SetStartSceneCommand,
 } from '../../core/commands/assetCommands';
@@ -214,6 +215,11 @@ export class AssetPanel extends Panel {
     if (this.state.execute(command)) this.state.openAsset(command.asset.id);
   }
 
+  private duplicateAsset(id: string): void {
+    const command = new DuplicateAssetCommand(id);
+    if (this.state.execute(command)) this.state.openAsset(command.copyId);
+  }
+
   /**
    * A sprite wearing the texture and an animation over it whose one row keys the frames in order,
    * one per frame – what a picture sequence is on a timeline. One undo step for all of it.
@@ -364,6 +370,7 @@ export class AssetPanel extends Panel {
       if (asset.type === 'pixelart' && asset.frames.length > 1)
         items.push({ label: 'Create animation from frames', onSelect: () => this.createAnimation(asset) });
     }
+    items.push({ label: 'Duplicate', onSelect: () => this.duplicateAsset(id) });
     items.push({ label: 'Delete…', danger: true, onSelect: () => confirmDeleteAsset(this.state, id) });
     const menu = new Menu(items);
     menu.open(x, y);