feat(cluster): add rolling-safe internode relay endpoints

[skhaz]

Summary

• Advertise an optional internode endpoint for nodes reached through a relay, proxy, or different externally visible port.
• Preserve the existing membership-IP plus internode_port metadata so older peers continue to connect during a rolling upgrade.
• Let upgraded peers select the advertised endpoint when both fields are present and valid; the legacy endpoint is used only when both new fields are absent.
• Fail closed on incomplete or malformed advertised metadata instead of silently dialing a different endpoint.
• Accept IP literals and DNS hostnames, with ports represented and validated separately.

The metadata keys are centralized as the wire contract shared by the boot publisher and internode consumer.

Validation

• go test -race ./cluster/internode ./boot/components/system
• make lint
• Tests cover legacy endpoint selection, advertised IP/DNS endpoint selection, incomplete metadata, malformed hosts, and malformed ports.