x402 Agent Pre-Trade Safety: ONE signed call returns a pre-execution risk verdict (SAFE/CAUTION/BLOCK) for payment/DeFi agents. Combines OFAC SDN/EU/UN/UK sanctions screening + stablecoin peg deviation (bps vs $1) + counterparty on-chain signal into one Ed25519-signed attestation. POST {address, chain?, counterparty?, stablecoin?}. Scoring/data only — never executes trades or moves funds.