feat: add mandatory signed gas-payment selectors to TxSeismic

[matthias-wright]

Adds GasPayment (Auto / Native / Token(address)) as a mandatory field of TxSeismic, authenticated by the transaction signature and independent of calldata encryption and AAD.

• Wire format: nested [kind, token] list immediately after gasLimit in RLP (Auto/Native carry an empty token), a nested GasPayment struct in the EIP-712 typed data (zero address for Auto/Native), and a tagged camelCase JSON representation
  (gasPayment) in requests and responses. No old-layout compatibility: pre-selector bytes are not accepted as a legacy format.
• Validation: Token(0x0) is invalid; decoders reject malformed selectors; requests default omission to Auto before signing.
• Requests/conversions: SeismicTransactionRequest.gas_payment with Reth and Foundry network conversions that preserve the selector through request → typed transaction → execution env → request round trips.
• Docs: docs/gas-payment.md specifies the layout and representations; README updated.
• Tests cover every variant across raw and EIP-712 signing, nonce and scalar boundaries, hashes, signer recovery and conversion round trips; these are the source of the shared gas-payment.json vectors consumed by the SDKs and Foundry.