v2.4.0: tab speaks x402

tab now speaks x402, the HTTP 402 payment standard Coinbase started and Cloudflare, Visa and Mastercard build on. Both directions.

x402 buyers can pay a tab gateway

• On the evm rail, every 402 carries x402 requirements (PAYMENT-REQUIRED, version 2, exact, EIP-3009) next to tab's own challenge.
• The gateway checks the signed transfer (amount, recipient, time window, signer, then a dry run on chain), calls the upstream, and settles from a small gas wallet (--x402-settler). The answer carries PAYMENT-RESPONSE and a signed tab receipt. If the transfer does not settle, the response is withheld.
• The public mainnet gateway takes x402 in USDG on Robinhood Chain (eip155:4663) now: all of its routes are open to any x402 agent. Its first x402 payment: 0x117e1566…e95e.

tab agents can pay x402 sites

• A site that answers with x402 only (version 2 header or version 1 body) is paid from the agent's own funded address, in USDG on Robinhood Chain or USDC on Base.
• The owner's mandate still binds it: hostname in allow, per-call cap, expiry, and the budget, counted by the client in x402-spent.json.
• tab evm fund --amount 5.00 --token 0x... funds an agent's address. A gateway that speaks both is paid the tab way.

Under the hood: src/x402.ts (wire format and EIP-712 signing, checked against the x402 spec's own example payment), spec draft 0.6 section 5.3, docs for agents and sellers. 282 automated checks (evm 59) plus 21 browser checks.