Contract-triad governance for the registry

Govern a mixed-format registry honestly: Avro subjects parse fully (records, enums, optionality, cross-subject references), JSON Schema subjects parse at depth 1 (properties, required, enum, format), and Protobuf subjects are inventoried with the protobuf-unparsed finding on every audit: the coverage gap never fades from memory. The loop closes with registry-ready .avsc generation from the governed model.