Function verify_tofu 

pub fn verify_tofu(
    kh: &mut KnownHosts,
    host: &str,
    port: u16,
    fingerprint: &str,
    replace: bool,
) -> SshCliResult<bool>

Verify TOFU fingerprint under exclusive flock (G-PAR-49 / G-TLS-10).

• No entry: accept and record (TOFU).
• Matching entry: accept.
• Differing entry: refuse, unless replace is true.

Reloads from disk under the lock so concurrent multi-host first-connect sees peers’ writes before deciding.

Errors

Returns an error if the host key changed and replacement was not allowed, or if persistence fails.