x402 payability preflight

Before an agent pays an x402 seller, GET /check?url=<seller> audits the seller's 402 challenge (wrong network, mainnet asset, bad payTo, price mismatch) and tells you what to fix. It costs $0.005 test USDC on Base Sepolia (eip155:84532).

1. Raw 402 challenge of /check: price, payTo, and the Bazaar discovery block, before anything is paid.

/health