CVE-2026-55678: arc has unauthenticated cluster node admission when `cluster.shared_secret` is unset

Arc Enterprise clustering accepts cluster join requests without authentication when cluster.enabled=true but cluster.shared_secret is not configured. The coordinator validates HMAC authentication only if a shared secret is non-empty; otherwise, a network attacker who can reach the coordinator port can send a join request with attacker-controlled node addresses and role. Accepted nodes are marked healthy, registered locally or added as Raft voters, and can be selected by the cluster router for forwarded authenticated requests.