SchemaRegistry

The documents $ref may reach, keyed by the URI each one publishes as its $id.

A catalog is not self-contained: nearly every property in one refers to common_types.json, and that document refers back into the catalog. The registry is what closes that loop, and holding the documents explicitly — rather than fetching them — is deliberate. A renderer resolving a $ref over the network would let an agent choose what its own payload is validated against.