Split Express entrypoint into focused modules

[mmurrs]

Summary

• keep src/express.ts as the public facade and createDual402 entrypoint
• move public/internal types into src/types.ts
• move startup config validation and x402 resolution into src/config.ts
• move charge middleware creation into src/charge.ts
• move paidRoute, dualDiscovery, and OpenAPI generation into src/discovery.ts

Why

This applies the maintainability review finding that src/express.ts had crossed 1k lines and mixed public API, config validation, payment middleware, and discovery generation in one file. The PR is intentionally mechanical so behavior and public imports stay stable.

The current handler metadata coupling and cast-heavy x402 shape probing are left unchanged for follow-up PRs.

Verification

• npm run check
• npm audit --omit=dev
• npm pack --dry-run