feat: add BSC (BNB Smart Chain) support to x402 — native + ERC-20 verification

[web3luka]

Add BSC (BNB Smart Chain) Support to x402

This PR introduces EVM network support for Binance Smart Chain (BSC) — including both native BNB and ERC-20 token verification (e.g., USDT / USDC).

It adds production-ready helpers under src/verify/evmVerify.ts, using the same model as other EVM integrations:

• verifyNativeExactByTxHash() — validates on-chain native transfers (BNB)
• verifyErc20ExactByTxHash() — validates ERC-20 token transfers
• verifyErc20ExactByLogs() — optional log-based scan mode for batch verification
• Fallback RPC clients with retry and timeout (withEvmClientsRetry)

---

Specification note

BSC currently does not support EIP-3009 (transferWithAuthorization) for its major stablecoins (USDT, USDC, etc.).
Therefore, this integration uses standard ERC-20 transfer verification, requiring users to submit and pay for gas themselves.

While this deviates from the fully gasless x402 payment model, it remains EVM-compliant and enables BSC ecosystem compatibility as an optional fallback network.

This PR introduces BSC support as an optional network extension.
While BSC currently lacks EIP-3009 support, standard ERC-20 flows remain valuable for ecosystem compatibility and can serve as a fallback model until gasless tokens (or wrapped variants) are available.

Checklist

• [x] I have formatted and linted my code (pnpm format && pnpm lint)
• [x] All new and existing tests pass (pnpm test)
• [x] My commits are signed (verified via git log --show-signature)

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel[bot]]

@web3luka is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.