Plug/Phoenix integration surface for HTTP/3 advertising, next to Bandit.

Bandit cannot terminate QUIC. Wire this into a Phoenix endpoint so H1/H2 responses tell clients where H3 lives (typically Caddy on UDP/443):

plug Gale.Plug.AltSvc, host: "example.com", port: 443

child_spec/1 starts nothing QUIC-related; it exists so an endpoint supervisor can list this adapter next to {Bandit, plug: ...} without forking Bandit.