Getting started with small-step operational semantics

What this post added

Introduces SagaLang, a minimal language for specifying sagas with forward and backward recovery, and uses Small-Step Operational Semantics (SOS) to formally define their execution. Details three iterations of SagaLang: basic execution, forward recovery with retries, and backward recovery with compensation. Highlights the developer's responsibility for ensuring idempotency and correct compensation logic for achieving transaction-like guarantees.