Function verify_proof_structure 

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

Verifies the structural rules of §7.1.11 and returns the mode the envelope is actually in.

This is the check that makes attestationMode mean something. It enforces, each with its own APH_E013 reason string:

• a chain carries EXACTLY two proofs;
• position 1 has proofPurpose == "assertionMethod", position 2 "authentication";
• both chain proofs carry an id, and the two ids differ;
• the notary proof’s previousProof equals the principal proof’s id — array position is a hint, this linkage is the binding (§7.1.11);
• the principal proof carries NO previousProof; it is the chain head;
• label and structure agree in BOTH directions: a chain MUST be labelled PrincipalSigned, and PrincipalSigned MUST be a chain. A mismatch is a rejection, never a silent coercion;
• the principal proof’s verificationMethod is a DID URL under credentialSubject.humanPrincipal.id. This is the rule that closes the forgery §7.1.11 describes, because the notary does not hold that key.

A single-object proof is a notary proof and yields NotaryAttested — the shape all eight published example envelopes carry.

A successful return says the structure is sound; it says NOTHING about whether either signature verifies. A caller that reports “the human signed this” on the strength of this function alone is reporting a claim no key has backed.