verify

Verify an engine change end-to-end by rendering the bundled fixture (examples/local-demo) and inspecting the output MP4 and intermediates. Use when the user says "verify", "run the smoke test", "e2e this change", "does the fixture still render", or before committing a nontrivial engine change. Knows the gotchas: OPENAI_API_KEY (or a local OPENAI_BASE_URL server) is needed for voice/captions but not…