feat(sdk): add Express x402 middleware

[zhailong8845-art]

Summary

• add x402Serve({ price, payTo }) as a route-level Express middleware helper
• configure Stellar testnet/pubnet verification and settlement through an x402 facilitator
• return the protocol 402 response for missing or invalid payment signatures
• validate configuration, cache route handlers, and document a runnable Express example

Tests

• npm test -- --runInBand — 5 tests passed
• npm run build — passed
• runtime export smoke — passed (x402Serve builds a middleware function)
• npm pack --dry-run — passed; source tests are excluded from the package

The repository lint script cannot run on the current baseline because ESLint is not declared or installed; this change does not introduce or alter that baseline script. npm also requires --legacy-peer-deps because the existing @stellar/mpp@0.4.0 peer range conflicts with the repository mppx@^0.5.5 declaration.

Closes #9

Bounty: GrantFox $25 USDC Express.js x402 middleware bounty in #9.

[zhailong8845-art]

Maintainer follow-up on the three submitted bounty PRs: #17, #18, and #19 are all ready for review, with the focused test evidence included in each PR.

Could you please confirm the acceptance order and the exact GrantFox payout step required after merge? I will address review feedback promptly. No bounty is being treated as earned or paid until your acceptance and an actual payout receipt.