Transfer a token from a signer without gas

The signer is the payer of an x402 exact payment: the gateway builds the token's EIP-3009 TransferWithAuthorization for exactly amount to to, the signer's key signs its EIP-712 digest, and the platform's facilitator verifies and settles it and pays the gas. The signer needs no native balance. One signing fee ($0.000005) is billed; no gas is. Idempotent on an optional Idempotency-Key with request fingerprinting: a matched replay returns the stored transfer, a mismatch answers 409 carrying the stored transfer's id, status and tx_hash, and a transfer that failed before settlement re-executes under the same key while one that died in settlement never does. Answers 200 settled with the transaction, or 202 unknown when the settle call's outcome never came back (the reconciler finishes it from the token's authorizationState; poll the transfer).