Close NAT channels after cancelling producers and quiet transient TUN backpressure

tests/test_nat_engine.c

@@ -1163,6 +1163,8 @@ static void test_hash_lifecycle(void) {
         ASSERT_EQ(eim_nat_egress(&ctx, packet, 42, 100000 + i, NULL), EIM_NAT_OK, "reuse freed port");
         uint16_t external; memcpy(&external, packet + 20, 2);
         ASSERT_EQ(be16toh(external), removed[i], "free ports reused in FIFO order");
+    }
+    for (size_t i = 0; i < 3; i++) {
         memcpy(packet, original, 42);
         ASSERT_EQ(eim_nat_egress(&ctx, packet, 42, removed[i], NULL), EIM_NAT_ERROR, "removed key does not alias reused slot");
         ASSERT(memcmp(packet, original, 42) == 0, "full-pool error preserves packet");