x402 security audit

Grade any x402 seller's payment-security posture from the outside - a read-only black-box check mapped to the 'Five Attacks on x402' failure modes. Probes the URL's 402 challenge (never pays) and scores TLS transport, gated-response cache hygiene (Attack III / cache leakage), error/info-leak hygiene, and payment-terms well-formedness, then returns a letter grade with per-check findings and an honest note on what only insider/active testing can confirm.