Function gather 

pub async fn gather(socks: &[UdpSocket], opts: &GatherOpts) -> Gathered

Work out every address a pilot could reach us on.

Mirrors the prototype’s gather_candidates: host candidates first (a LAN address wins instantly on a shared network; global IPv6 has no NAT at all), then an explicit router port mapping, then the STUN-reflexive address — suppressed under symmetric NAT, where it can only waste a slot in the pilot’s race. socks[0] must be the IPv4 socket QUIC will use.