validate_adjudication_fragment contract

Trap: validating a one-record fragment against the canonical adjudication envelope looks right because its reviewer and record are schema-valid, but the canonical schema requires envelope-wide fields such as adjudication_id and the frozen 40-record set. Fix: require exactly schema_version, design_status, reviewers, and nonempty records, then validate each reviewer and record against $defs.reviewer and $defs.record; never inspect existing fragments merely to infer this contract.