```

feat(cache): 实现并发安全的 DNS 响应缓存机制

新增基于 sync.Map 的缓存结构，支持 TTL 管理与定时清理。
完善缓存键生成逻辑，包含 DO/CD 标志避免上下文污染。
增强缓存读写处理：
- 自动跳过伪 RR（OPT/TSIG）防止干扰 TTL 计算
- 写入前剥离传输层细节提升通用性
- 支持负面缓存（NXDOMAIN/NODATA）并遵循 RFC 2308
- 回填剩余 TTL 并确保对外不可变

优化上游查询模块：
- 并发向多个上游竞速，支持 UDP 截断后 TCP 回退
- 过滤不良 RCODE（SERVFAIL/REFUSED/FORMERR）提升稳定性
- 使用信号量控制最大并发数，改善资源利用率
- 快速失败机制减少无效等待

其他改进：
- 完善日志记录，区分缓存命中/未命中及响应内容
- 显式构建客户端兼容的 EDNS0 选项
- 增加注释说明关键设计决策和行为边界
```

go.sum

@@ -1,12 +1,35 @@
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