Add IPv6 client support with Happy Eyeballs

[benoitc]

Adds IPv6 to the client connect path.

quic:connect/4 now accepts IP-tuple and bracketed IPv6 literal hosts, resolves hostnames in the caller process (returning {error, _} instead of silently dialing localhost on failure), and uses RFC 8305 Happy Eyeballs for dual-stack hostnames: addresses are raced IPv6-first and the first to finish its handshake is returned. For a multi-address hostname connect/4 blocks until the first attempt connects; a single address, IP literal/tuple, or pre-opened socket keeps the immediate async return.

The migration rebind now reopens the socket with the connection's address family. Racing attempts and coordinators run under supervision, and connections monitor their owner so a supervised winner closes when its owner exits.

New options: happy_eyeballs (default true), family (inet | inet6 | any), connection_attempt_delay (250 ms), connect_timeout (5000 ms).

quic:connect("example.com", 443, #{}, self()).                 % Happy Eyeballs, IPv6-first
quic:connect("[::1]", 443, #{}, self()).                       % IPv6 literal
quic:connect("example.com", 443, #{family => inet6}, self()).  % force IPv6