Function require_mode 

pub fn require_mode(
    envelope: &NotarizationEnvelope,
    required: AttestationMode,
) -> Result<(), AphError>

Refuses an envelope weaker than the caller’s policy demands (§8.3.1 1a).

PrincipalSigned required against a NotaryAttested envelope is APH_E012. The reverse is fine: a stronger envelope satisfies a weaker policy. There is no silent downgrade, for the same reason §8.4.6 forbids downgrading key discovery — an attacker who can defeat the weak path will always present the weak path.

This reads the DECLARED label, and deliberately so: §8.3.1 puts it at step 1a precisely so a verifier refuses a too-weak claim before doing work on unauthenticated input. The label is not evidence on its own, so a caller MUST also call verify_proof_structure, which is what rejects a PrincipalSigned label written above a structure that does not support it. Calling this function alone accepts a forged label.