The legacy DNS resolver path creates NAT pairs with destination
0.0.0.0/0 (a prefix, not a DomainSet). The v6 NAT duplication only
triggered for DomainSets, so legacy dynamic routes never got a v6
NAT rule.
Extract NeedsV6NATDuplicate and ToV6NatPair helpers that detect both
DomainSets and the v4 default wildcard 0.0.0.0/0. Both nftables and
iptables managers now use these for Add/RemoveNatRule, ensuring v6
NAT duplication works for both modern and legacy DNS resolver paths.