Artifact: RFC 2119 Contract — Shipment Event Schema

Shipment Event Schema contract, rewritten with RFC 2119 keywords -- exercises co-13.

Every event MUST carry a globally unique event_id. Consumers MUST implement idempotent processing keyed on event_id, because the underlying queue provides at-least-once delivery, not exactly-once. Consumers MUST NOT assume the payload's field list is closed -- new optional fields MAY be added in the future, and a conformant consumer SHOULD ignore fields it does not recognize rather than rejecting the event.