feat(proxy): 安全加固与架构优化

- 引入 SafeDnsResolver 消除 DNS rebinding TOCTTOU 漏洞，DNS 缓存改为多地址存储
- 扩展私有 IP 检测范围（CGNAT 100.64/10、基准测试 198.18/15、保留 240/4）
- 请求处理增加 hop-by-hop 头过滤、URL scheme 校验、超时范围限制
- 动态配置从 RwLock 切换到 ArcSwap 实现无锁读取
- 启动注册失败的服务器支持后台自动重试
- WebSocket 帧大小上限提升至 64MiB 匹配 Python 端
- 心跳支持动态间隔更新，新增 failed_requests/dns_failures/stream_errors 指标
- 配置启动校验、systemd UMask=0077、配置文件权限 600
- Python 端支持 per-connection max_streams（X-Tunnel-Max-Streams）

aether-proxy/Cargo.lock

@@ -13,6 +13,7 @@ name = "aether-proxy"
 version = "0.2.0"
 dependencies = [
  "anyhow",
+ "arc-swap",
  "base64",
  "bytes",
  "clap",
@@ -111,6 +112,15 @@ version = "1.0.101"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5f0e0fee31ef5ed1ba1316088939cea399010ed7731dba877ed44aeb407a75ea"
 
+[[package]]
+name = "arc-swap"
+version = "1.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9f3647c145568cec02c42054e07bdf9a5a698e15b466fb2341bfc393cd24aa5"
+dependencies = [
+ "rustversion",
+]
+
 [[package]]
 name = "atomic"
 version = "0.6.1"