[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.

まず既存の HelloSaga と MoneyTransfer のサンプルを読んで、SAGA と compensation がどのように示されているかを理解します。要求された 7 つの状態、idempotency の処理、retry backoff、child-workflow の compensation を中心にサンプルを定義します。1 つの一貫した payment workflow で 4 つすべての動作を示せれば完了です。