[Sample Request] Payment workflow with idempotency, state machine, and compensation flow

Hi, I'd like to contribute a new sample demonstrating a production-grade payment workflow pattern.

What it would cover:

• 7-state payment state machine (INITIATED → VALIDATING → PROCESSING → COMPLETED / FAILED / CANCELLED / COMPENSATING)
• Idempotency key deduplication before any state mutation
• Exponential backoff with jitter on retries
• Compensation flow as a child workflow on failure

Why it's useful:
The existing HelloSaga and MoneyTransfer samples demonstrate SAGA and basic compensation, but none show the full combination of idempotency + state machine transitions + compensation in a single cohesive example. This pattern is extremely common in fintech/payment backends.

Happy to submit a PR if this is welcome. Let me know if there's a preferred package structure or naming convention I should follow.

Commencez par lire les exemples existants HelloSaga et MoneyTransfer afin de comprendre comment SAGA et la compensation sont démontrés. Définissez l’exemple autour des sept états demandés, de la gestion de l’idempotence, du backoff des retries et de la compensation des child-workflows ; il est terminé lorsqu’un workflow de paiement cohérent démontre les quatre comportements.