The XHTTP inbound set every accepted connection's LocalAddr to the
listener's address (h.localAddr = l.listener.Addr()). On a wildcard
listener that is the unspecified address ("[::]" / "0.0.0.0").

sendThrough "origin" derives the outbound gateway from inbound.Local,
which comes from conn.LocalAddr(). So with XHTTP every connection's
egress was bound to the wildcard, collapsing all entry IPs onto one
(often IPv6) source address and breaking source-in-source-out on
multi-IP hosts (and failing outright when that address has no route).

Read the concrete per-connection local address from the request context
(http.LocalAddrContextKey), which net/http populates with the address
the client actually connected to. Fall back to the listener address when
the key is absent (e.g. HTTP/3, where net/http does not set it).