Fix smoke test shadow fn to return a function

Same fix as the integration test — the shadowed fn must return a
function since its return value is passed to the on modifier.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NullVoxPopuli

smoke-tests/scenarios/basic-test.ts

@@ -393,6 +393,7 @@ function basicTest(scenarios: Scenarios, appName: string) {
                   // shadows keyword!
                   const fn = () => {
                     assert.step('shadowed:fn:invoke');
+                    return () => {};
                   };
 
                   class Demo extends Component {