Function verify 

pub fn verify<R: Read>(
    data: &mut R,
    armored_sig: &str,
    signer_identity: &str,
    namespace: &str,
    allowed: &AllowedSigners,
) -> Result<Verified, GitwayError>

Verifies that armored_sig is a valid SSHSIG over the bytes read from data, in namespace, and that allowed authorizes signer_identity to sign with the embedded public key.

This is the full ssh-keygen -Y verify equivalent: three independent checks — cryptographic signature, namespace match, and principal authorization.

Errors

Returns GitwayError::signature_invalid on any failed check.