IPv6 support (roadmap 1.4)

[cvik]

Summary

Removes all IPv4-only hardcoding. Server and client auto-detect address family from bind/host address string.

Implementation

• Io.zig: socket family from bind address, dual-stack IPV6_V6ONLY=0, family-aware decode_recv
• Server.zig: sockaddr.in6 recv buffer, std.net.Address response storage, family-based msg_recv.namelen
• Client.zig: removed AF_INET check, socket from parsed family
• exchange.zig: family-aware FNV-1a hashing for addr_hash/peer_key
• rate_limiter.zig: new AddrKey type replacing ip_addr: u32
• DTLS Cookie + Session: fixed address truncation for IPv6
• Bench: --ipv6 flag switches all scenarios to ::1/::

Test plan

• All 120+ tests pass (including new IPv6 round-trip test)
• IPv6 bench: --ipv6 --plain-only --non-only runs with 0 errors
• IPv4 bench unchanged