InstanceConfig was a ~50-field struct designed for Wild Cloud k8s instances,
but only 3 fields were ever read by dnsmasq (the sole consumer). The
reconciliation bridge constructed fake InstanceConfig objects from registered
domains just to satisfy this interface.
Replace with DNSEntry{Domain, IP} — a 2-field struct purpose-built for
dnsmasq. All domains get local=/ directives to prevent AAAA queries from
leaking to upstream DNS (Happy Eyeballs / RFC 8305 latency on LAN).
Removed dead code: InstanceConfig, NodeConfig, LoadCloudConfig,
SaveCloudConfig, DeepMerge, LoadMergedInstanceConfig, EnsureInstanceConfig,
instance path helpers, instanceLoadBalancerIP, GenerateInstanceConfig,
and all modular per-instance dnsmasq methods.