Plugins in data/plugins are loaded at startup and can add a tab with a read-only node board, and the demo plugin shows every asset as a node (#113)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/lib/scaffold.mjs

@@ -112,6 +112,8 @@ export const TAB_LABELS = {
   container: 'Container',
   list: 'List',
   particles: 'Particles',
+  // Added at start-up by the demo plugin (plugins/demo-plugin.js), after every built-in tab.
+  'plugin:assets': 'Assets',
 };
 
 /**