feat(net): Phase 7 — dual-socket IPv4+IPv6 ICE

Adds a dedicated IPv6 QUIC endpoint (IPV6_V6ONLY=1 via socket2)
alongside the existing IPv4 signal endpoint for proper dual-stack
P2P connectivity. Previous [::]:0 dual-stack attempt broke IPv4
on Android; this uses separate sockets per address family like
WebRTC/libwebrtc.

- create_ipv6_endpoint(): socket2-based IPv6-only UDP socket,
  tries same port as IPv4 signal EP, falls back to ephemeral
- local_host_candidates(v4_port, v6_port): now gathers IPv6
  global-unicast (2000::/3) and unique-local (fc00::/7) addrs
- dual_path::race(): A-role accepts on both v4+v6 via select!,
  D-role routes each candidate to matching-AF endpoint
- Graceful fallback: if IPv6 unavailable, .ok() → None → pure
  IPv4 behavior identical to pre-Phase-7

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-transport/src/lib.rs

@@ -23,7 +23,7 @@ pub mod quic;
 pub mod reliable;
 
 pub use config::{client_config, server_config, server_config_from_seed, tls_fingerprint};
-pub use connection::{accept, connect, create_endpoint};
+pub use connection::{accept, connect, create_endpoint, create_ipv6_endpoint};
 pub use path_monitor::PathMonitor;
 pub use quic::QuinnTransport;
 pub use wzp_proto::{MediaTransport, PathQuality, TransportError};