Function verify_certificate 

pub fn verify_certificate(cert: &ProofCertificate) -> bool

Verify a proof certificate (check that steps are internally consistent).

Performs the following checks:

1. Certificate has at least one step
2. Certificate is marked checkable
3. No forward references in premise indices
4. Claim binding integrity: steps bound to the claim must match the current claim digest
5. Conclusion consistency: the final step’s conclusion must agree with the claim type
6. Formula chain integrity: formula digests are verified against stored formulas