Validates a signed x402 payment authorization without moving any funds: it checks the Ed25519 signature, freshness, nonce, balance, and a simulated transfer, then reports whether the authorization is acceptable. Use it to confirm a payment is good before anyone commits funds, or to poll until a submitted on-chain transaction lands. Requires a signed Authorization header; the x402 authorization travels in the request body (a paid resource that issued the 402 challenge expects it via the X-Payment header).