Gate a model with an evaluation card

Reinforce model-card / NIST AI RMF order from ML-governance training. Production traffic is not pointed at a new model until the card, fairness slice, and rollback are signed.

1. Fill the model card: data, intended use, and out-of-scope uses.
2. Report overall and slice metrics against the gate.
3. Document known failure modes and human oversight.
4. Stage behind a flag with a rollback.
5. Obtain the review signatures.
6. Do not silently replace a production model from a notebook.