Asset panel: double-click opens the tab, single click only selects (#71)

One rule for every asset type: a click on an asset row selects it, a
double-click (or Enter, or creating it via +) opens its tab. Selection
alone still switches tabs only for nodes, frames and template objects.

- state.openAsset(id) = select + 'open' event; the editor maps the asset
  type to its tab via TAB_OF_ASSET. The two special clauses for state
  machine and shader assets in the selection handler are gone.
- Creating via + and importing a texture from the picker open the tab;
  paste/drop of an image only selects.
- 3 unit tests (event order, re-open of a selected asset, selection never
  fires open), 14 e2e checks in step 4.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/editor/open-asset.test.ts

@@ -0,0 +1,43 @@
+/**
+ * Opening an asset (double-click, Enter, creating one) is selecting it plus a request for its tab.
+ * The editor switches on `open`, never on `selection` alone – so the order and the "already
+ * selected" case are what matter here.
+ */
+import { describe, expect, it } from 'vitest';
+import { CreateAssetCommand } from '../../src/core/commands/assetCommands';
+import { createEmptyProject } from '../../src/core/model/Project';
+import { EditorState } from '../../src/editor/state';
+
+function setup(): { state: EditorState; assetId: string; events: string[] } {
+  const state = new EditorState();
+  state.loadProject(createEmptyProject('p', 'P'), 1);
+  const create = new CreateAssetCommand('behavior', 'B');
+  expect(state.execute(create)).toBe(true);
+  const events: string[] = [];
+  state.on('selection', () => events.push('selection'));
+  state.on('open', () => events.push('open'));
+  return { state, assetId: create.asset.id, events };
+}
+
+describe('opening an asset', () => {
+  it('selects it first and asks for the tab after, so the tab finds the selection in place', () => {
+    const { state, assetId, events } = setup();
+    state.openAsset(assetId);
+    expect(state.selection).toEqual({ kind: 'asset', assetId });
+    expect(events).toEqual(['selection', 'open']);
+  });
+
+  it('still asks for the tab when the asset was selected already – a click did not open it', () => {
+    const { state, assetId, events } = setup();
+    state.select({ kind: 'asset', assetId });
+    events.length = 0;
+    state.openAsset(assetId);
+    expect(events).toEqual(['open']);
+  });
+
+  it('never fires on a plain selection', () => {
+    const { state, assetId, events } = setup();
+    state.select({ kind: 'asset', assetId });
+    expect(events).toEqual(['selection']);
+  });
+});