Validate a US bank routing number (ABA) deterministically: 9-digit check, ABA mod-10 weighted checksum, Federal Reserve district decode and institution-id extraction. Zero external dependencies, no hallucination, always available. Use it to validate a US bank account before an ACH/wire payment. Complements IBAN validation for international. Ed25519-signed. 0.001 USDC on Base. First 3 calls FREE per wallet — send header X-WALLET: 0x<your Base address> (no payment); then per call.