Context

Work was driven by the hole-punch-interop harness in libp2p/test-plans (Docker + rust-libp2p relay + artificial latency). TCP hole-punch paths could fail against the relay; QUIC showed handshake timeouts and aioquic logger errors.

Problems addressed

1. Circuit relay v2 (client): Some relays (e.g. rust-libp2p) treat one HOP substream as a single exchange and close it after RESERVE. The client previously reused the same stream for CONNECT, which led to dropped streams / failed relay dials. After a successful client reservation, the transport should use a new stream to the relay for CONNECT.

2. QUIC dial: The cached QuicConfiguration was mutated per dial (is_client, QuicLogger), causing shared-state races and aioquic errors such as QuicLoggerTrace does not belong to QuicLogger. Each dial should use a copy of the client config; tracing is disabled on that path (quic_logger = None) unless a per-dial logger is desired.

3. Circuit v2 protobuf / discovery / tests: Align with current circuit v2 usage and update tests accordingly (including destination reservation behavior for CONNECT).

Follow-up

Harness-side changes (compose timeouts, test ordering, docs) remain in test-plans and are tracked separately.