Publish private DNS names for compute instances

[scotwells]

Summary

Publish private DNS names for instances when InternalDNSPublishing is enabled; the feature is disabled by default. Each instance uses its name in the network's managed zone, such as web-01.datum.internal, and reports assigned names and publication readiness by network name and namespace. Confirmed endpoint health changes update eligibility; failed or unknown observations retain existing records and report unknown readiness. Instance deletion withdraws records.

Test plan

• CI: Publisher lifecycle and feature gate
• CI: Unknown observations preserve publication state
• CI: Explicit health withdrawal and stable hostnames
• CI: Namespaced status, scoped grants, and availability patches preserve DNS settings