docs: 补充 E2E 测试策略说明
基于 Playwright MCP 实践添加测试策略要点:
- 即时验证:Story 开发过程中立即验证
- 模块化验证:每完成一个模块立即验证
- 减少返工:早期发现问题减少成本
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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>