Give the e2e steps one place to build their fixtures

Steps 18 and 20 both hand-rolled the same object-plus-behavior scaffold, down
to lane offsets that nothing kept distinct. Step 19 claimed in a comment that it
cleaned up after itself but never checked it – the only one of the four that
did not.

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

tools/e2e/19-nested-behavior.mjs

@@ -297,4 +297,13 @@ export async function nestedBehavior(t) {
     scene.objects = scene.objects.filter((o) => o.id !== 'nb-box' && o.id !== 'nb-logic');
   `);
   await sleep(300);
+  const left = await json(`{
+    objects: __gm.sceneObjects().filter((o) => o.id.startsWith('nb-')).length,
+    assets: __gm.state.project.assets.filter((a) => a.id.startsWith('nb-')).length,
+  }`);
+  check(
+    'the step leaves nothing of its own behind',
+    left.objects === 0 && left.assets === 0,
+    JSON.stringify(left),
+  );
 }