9420ad946a
On Linux, SO_REUSEADDR only allows rebinding past TIME_WAIT (a second live listener would need SO_REUSEPORT, which we never set), so disabling it bought no protection there while making a quick gateway restart fail to rebind for up to ~60s. Keep the BSD silent-split guard on darwin, default semantics elsewhere. E2E verified: dual-stack v4+v6 bind on one port, immediate rebind after disconnect, and live-listener conflict still rejected.