feat(x402): BSC multi-asset payments + Permit2 signing

[gulshngill]

What

The Nansen API now advertises four stablecoins on BNB Smart Chain — U (United Stables), USD1, USDT, and USDC. This PR makes the CLI able to pay with all of them.

U and USD1 already work with the CLI's existing EIP-3009 flow (the server lists them first for exactly that reason). USDT and USDC on BSC predate EIP-3009, so facilitators settle them through Uniswap's canonical Permit2 contract — which needs a different signature type.

Changes

src/x402-evm.js — Permit2 signing (permit2-exact)

• Payment creation routes on the 402's extra.assetTransferMethod: absent/eip3009 → existing gasless flow; permit2-exact → new createPermit2ExactPayload; anything else throws so the fallback loop moves to the next accepts entry.
• Signs a PermitWitnessTransferFrom against the spender contract advertised in the 402 (extra.spenderAddress — never hardcoded), witness bound to the merchant wallet. Nested-struct EIP-712 hashing and the 3-field Permit2 domain are implemented in the repo's existing zero-dependency style.
• Cross-validated: the digest test vector was computed with an independent EIP-712 implementation (Python eth_account) over identical input — the two agree byte-for-byte.

src/x402.js — allowance preflight + multi-asset registry

• Permit2 requires a one-time on-chain token.approve(PERMIT2_ADDRESS, …) from the payer. Entries are skipped with an actionable message when the allowance is missing, instead of burning a doomed verify round-trip. RPC failures fail open (server decides).
• EVM_X402_TOKENS becomes an ordered per-network token list (BSC: U, USD1, USDT, USDC — all 18-decimal BEP-20 deployments); RPC URLs split into EVM_X402_RPCS.
• checkX402Balance(network, asset) checks the token actually paid with, so low-balance warnings use the right symbol and decimals.

src/api.js — threads the paid asset through to the balance warning.

Testing

• New x402-permit2.test.js (7 tests): reference digest vector, payload wire shape (decimal-string numerics, no EIP-3009 fields), missing-spender error, method routing (permit2-exact / eip3009 / absent / unsupported).
• Registry tests updated for the multi-asset shape.
• Full suite: 1498 passed.

Notes

• Wallets holding U or USD1 pay on BSC with no extra steps. Wallets paying USDT/USDC on BSC need the one-time Permit2 approval (requires a little BNB for that single transaction; every payment after is a gasless signature).
• permit2-upto is intentionally unsupported for now — the CLI only uses exact-amount payments.

🤖 Generated with Claude Code

[kome12]

Minor: this preflight only detects the zero-allowance case (> 0n), but the skip message below tells users to approve(Permit2, <amount>). A wallet that approved a small finite allowance now exhausted (or below the payment amount) would pass this check and then fail server-side instead of getting the actionable skip message.

Since it fails open on RPC error anyway, the heuristic is defensible — but consider either a one-line comment noting it only catches the never-approved case, or threading the payment amount through to compare allowance >= amount. Non-blocking.

[gulshngill]

Good catch — went with the real fix rather than the comment: hasPermit2Allowance now takes the payment amount and compares allowance >= amount, so the exhausted/insufficient-finite-approval case gets the actionable skip message too. Skip message updated to name both cases. Fail-open on RPC error unchanged. Fixed in d02572c.