refactor: extract internal process-event module and split tests

- Move processEvent implementation to process-event.ts (not re-exported from package entry)
- Add processor-defaults.ts for shared defaults/backoff to avoid import cycles
- Add processor-event.test.ts for direct processEvent coverage
- Extend processor integration tests (wakeup, poll limits, shutdown timeout)
- Keep defaultBackoff public API via processor re-export

Author: adavila0703