Module step 

Step operation — checkpointed work units with optional retries.

Implement FR8, FR9, FR10, FR11: named steps that execute a closure, checkpoint the result, support server-side retries, and handle typed errors.

This module follows the Python SDK’s two-phase checkpoint pattern:

1. Checkpoint START (sync)
2. Execute closure
3. Checkpoint SUCCEED, FAIL, or RETRY (sync)