MNHA: Redundancy domain: Domain-local IPsec VPN

Four-node Multinode High Availability supports multi SRG1 (SRG1+) within domains, enabling IPsec VPN in the deployment. In this setup, IPsec VPN tunnels are scoped per domain and do not span the entire deployment. Each domain operates as an independent two?node MNHA IPsec pair, and IKE/IPsec security associations synchronize only within that domain. This design eliminates cross-domain state sharing by scoping IPsec SAs per domain, ensuring predictable per-domain failover.