com.fedproxy.temp.xrpc.subscribe

Open a relay channel. The subscriber presents a com.fedproxy.temp.xrpc.registration: a record signed (network.attested.signature) over the nonce previously issued by com.fedproxy.temp.xrpc.getRegistrationNonce. The relay verifies the registration's signatures against the issued nonce and the did:key, then binds the WebSocket to that key. On connect the relay sends a #registered frame with the assigned serviceId/proxyRef; subsequent #request frames carry inbound XRPC calls the subscriber answers with #response frames.