Payment security firewall scan for an OUTGOING x402 payment: PII/secret leak detection in payment metadata, nonce replay detection, overpayment detection, prompt-injection-triggered payment analysis (fast + deep tiers), canonical-USDC asset verification, merchant pinning, velocity/spend caps, and counterparty reputation cross-check. Returns allow/flag/block with per-check reasons and an Ed25519-signed verdict. Advisory and non-custodial.