perf(transport): Optimize protocol name construction on substream open

[dimartiro]

Summary

Protocol negotiation lists were rebuilt on every substream open even though they are static after handler initialization. Precompute them once and share via Arc:

• TransportService precomputes Arc<[ProtocolName]> for outbound protocols; ProtocolSet exposes Arc<HashMap<...>> for the keep-alive map.
• ProtocolCommand::OpenSubstream carries the shared Arc instead of the duplicated (protocol, fallback_names) pair.
• TCP/QUIC/WebSocket negotiate_protocol is generic over IntoIterator and iterates the Arc'd lists directly.
• WebRtcDialerState stores Arc<[ProtocolName]> + cursor; webrtc_listener_negotiate accepts IntoIterator<Item = &ProtocolName>.

Closes #346

[lexnv]

@dimartiro Thanks a lot for contributing!

This conflcts pending webrtc work, we'll dedicate a separate review once we stabilize the transport layer