After ~90s of unanswered handshake initiations (the give-up branch of
expiredRetransmitHandshake) the socket's 5-tuple is proven dead (expired
NAT mapping / poisoned DPI flow entry after device sleep) and upstream
retries into it forever; only a manual reconnect healed the peer.
Reopen the bind once per give-up cycle (fresh ephemeral port unless the
user pinned listen_port), then re-initiate immediately. Debounced via CAS;
no timers or goroutines while healthy; a rebind racing Down()/Close()
degrades to a no-op inside BindUpdate. Red/green e2e + unit tests.
