lx: rebind socket on handshake give-up (sing-box-lx SPEC 041 self-heal)

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.

device/lx_giveup_rebind_test.go

@@ -0,0 +1,83 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * lx: SPEC 041 — unit tests for the give-up rebind mechanics on top of the
+ * self-heal harness (lx_giveup_selfheal_test.go): fresh vs pinned port,
+ * debounce, and disabled = upstream parity. These use the post-fix API
+ * (SetGiveUpRebind) and are NOT expected to compile on the pre-fix base.
+ */
+
+package device
+
+import (
+	"testing"
+	"time"
+)
+
+func waitOpens(t *testing.T, bind *gateBind, want int) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for bind.openCount() < want {
+		if time.Now().After(deadline) {
+			t.Fatalf("bind reopened %d times, want %d", bind.openCount(), want)
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
+// Fresh mode (listen_port not pinned): the rebind must ask the OS for a new
+// ephemeral port — Open is called with port 0.
+func TestGiveUpRebindFreshPort(t *testing.T) {
+	pair := newGiveUpPair(t, false)
+	pair.devA.SetGiveUpRebind(true, true)
+
+	triggerGiveUp(t, pair.devA, pair.pkB)
+	waitOpens(t, pair.bindA, 2)
+
+	ports := pair.bindA.portsSnapshot()
+	if ports[1] != 0 {
+		t.Fatalf("rebind requested port %d, want 0 (fresh ephemeral)", ports[1])
+	}
+}
+
+// Pinned mode (explicit listen_port): the rebind must keep the current port.
+// chanBind.Open reports its source id (1) as the actual port, so the device
+// stores net.port=1 after the first Open and must reuse it.
+func TestGiveUpRebindPinnedPortPreserved(t *testing.T) {
+	pair := newGiveUpPair(t, false)
+	pair.devA.SetGiveUpRebind(true, false)
+
+	triggerGiveUp(t, pair.devA, pair.pkB)
+	waitOpens(t, pair.bindA, 2)
+
+	ports := pair.bindA.portsSnapshot()
+	if ports[1] != 1 {
+		t.Fatalf("rebind requested port %d, want 1 (pinned)", ports[1])
+	}
+}
+
+// A second give-up inside the debounce window must not rebind again.
+func TestGiveUpRebindDebounce(t *testing.T) {
+	pair := newGiveUpPair(t, false)
+
+	triggerGiveUp(t, pair.devA, pair.pkB)
+	waitOpens(t, pair.bindA, 2)
+
+	triggerGiveUp(t, pair.devA, pair.pkB)
+	time.Sleep(300 * time.Millisecond)
+	if got := pair.bindA.openCount(); got != 2 {
+		t.Fatalf("debounce failed: bind opened %d times, want 2", got)
+	}
+}
+
+// Disabled: the give-up branch must behave exactly like upstream — flush and
+// stop, no rebind.
+func TestGiveUpRebindDisabled(t *testing.T) {
+	pair := newGiveUpPair(t, false)
+	pair.devA.SetGiveUpRebind(false, false)
+
+	triggerGiveUp(t, pair.devA, pair.pkB)
+	time.Sleep(300 * time.Millisecond)
+	if got := pair.bindA.openCount(); got != 1 {
+		t.Fatalf("disabled mechanism still rebound: %d opens, want 1", got)
+	}
+}