Function detect_identity_fork 

pub fn detect_identity_fork(
    old_identity: &str,
    new_identity: &str,
    has_history: bool,
    acknowledged: bool,
) -> Option<ValidationError>

Pure decision for the fork-on-edit guard on a username@hostname change. Returns Some(IdentityWouldFork) iff the policy has snapshot history, the change was not acknowledged, and the resolved identity actually differs. The webhook does the IO (read the old object’s pinned identity + history, resolve the new identity) and calls this.

use kopiur_api::validate::detect_identity_fork;

// History + a real change + no ack → fork.
assert!(detect_identity_fork("pg@billing", "pg@payments", true, false).is_some());
// No history yet (e.g. typo fixed before the first backup) → allowed.
assert!(detect_identity_fork("pg@billing", "pg@payments", false, false).is_none());
// Acknowledged → allowed.
assert!(detect_identity_fork("pg@billing", "pg@payments", true, true).is_none());
// No actual change → allowed.
assert!(detect_identity_fork("pg@billing", "pg@billing", true, false).is_none());