Protobuf codec for the Hrana 3 over WebSocket messages: ClientMsg (hello / request) and ServerMsg (hello_ok / hello_error / response_ok / response_error), plus the RequestMsg / ResponseOkMsg request- and response-oneofs. Maps to and from the same message maps Filo.Socket already dispatches on, so the socket only swaps text-JSON framing for binary-protobuf framing. Shared structures come from Filo.Protobuf.