Pay per call from a browser wallet

[lightchainaidev]

Per-call payment used to take only a local account (a raw private key). It now also takes a browser wallet's JSON-RPC account, { address, type: 'json-rpc' }, with the wallet as transport (custom(window.ethereum)). A web page can then pay each call through the SDK, with the visitor's own wallet signing.

• Signing goes through the client's wallet client. A local account still signs by itself, byte for byte as before. A JSON-RPC account asks its wallet for one eth_signTypedData_v4. No transaction, no gas.
• Smart accounts stay out. The JobRegistry takes only an ECDSA signature by the payer's own key.
• A rejected signature comes back as the 402, its error.message starting Not paid by the SDK: User rejected the request., followed by the wallet's own reason.
• README: a "From a browser wallet" section:
  • how to set it up;
  • put the wallet on the network's chain first;
  • what a rejected signature returns;
  • the authorization's clock;
  • one address per client;
  • no smart accounts.

Tests

npm run typecheck, npm run build and npm test all pass (73 of 73).

• A stand-in browser wallet signs all 9 vectors the SDK signs, and the payment must match each vector byte for byte. It must arrive as exactly one eth_signTypedData_v4 for the payer, with no transaction.
• A wallet that rejects (code 4001, MetaMask's wording) gets the 402 back. The message carries both viem's prefix and the wallet's reason, and the request is not sent again.

Not covered: deposit() through a browser wallet, and a real wallet against the live API.