Provider profile client protocol

Fetch capabilities before registration. For direct_server, seal a credential with the advertised public key, binding the client identity, key ID, issued time and a fresh nonce. Never reuse an envelope after a registration failure; create a fresh nonce and envelope. Store only the returned provider_profile_id and non-secret display metadata. Review submissions select that identifier alone. Handle provider_profile_unavailable without exposing credentials or endpoints. See OpenAPI and the executable synthetic protocol fixtures.