- UDP proxy: datagram relay client↔exit over ETCP
- ICMP proxy: echo ping relay client↔exit over ETCP
- Move via_node_id from per-forward-mapping to global tcp_proxy setting
- Support multiple concurrent tcp_proxy instances (no singleton g_tcp_proxy)
- Cap handshake padding to stay within 1472+overhead
- Handle non-TCP packets (UDP/ICMP) in tcp_proxy raw/tun input paths
- Add utun_instance_create_from_str() for test config creation
