Function verify_pull 

pub fn verify_pull(
    entry: &RegistryEntry,
    frontier_path: &Path,
) -> Result<(), String>

Pull verification: given a registry entry and the path to a pulled-frontier file on disk, verify that:

1. The entry’s signature verifies against its declared pubkey.
2. The frontier’s snapshot_hash matches the entry’s latest_snapshot_hash.
3. The frontier’s event_log_hash matches the entry’s latest_event_log_hash.

Returns Ok(()) if all three hold; Err(reason) on any mismatch.