engine-sanity

The mandatory pre-flight contract checks (Stage S0) that must pass before trusting NexusTrade's certification engine for any strategy work. Use at the start of a bakeoff or certification campaign to prove the walk-forward engine runs end-to-end, windows don't leak, fold winners persist, and the engine doesn't fabricate values for dead/not-yet-listed names. Each check has a STOP-and-report failure mode; these runs do…