feat: complete epic 1 API compatibility

[mrnicholasbcarter-code]

Summary

• add exported Next.js /api-compatible handler over existing gateway pipeline
• document Express peer boundary, Next.js usage, and fail-open behavior
• add route-level tests for Next.js-like POST and non-POST handling

Verification

• npm test -- --runInBand
• npm run build
• npm run typecheck
• npm run format:check
• npm run verify:package
• git diff --check

Closes #1