Function verify_path_signatures 

pub fn verify_path_signatures<F>(
    path: &CertPath<'_>,
    verifier: F,
) -> VerificationResult
where
    F: Fn(&[u8], &[u8]) -> Result<(), String>,

Hook for signature verification — caller provides a verifier function. The verifier receives (parent_pubkey, signed_cert_der) and returns Ok(()) if the signature is valid.