[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.

首先阅读现有的 HelloSaga 和 MoneyTransfer sample，以了解 SAGA 和 compensation 的演示方式。围绕所要求的七个状态、幂等性处理、retry backoff 以及 child-workflow compensation 定义该 sample；当一个完整统一的 payment workflow 演示出全部四种行为时，即表示完成。