feat(socket): add Linux SO_MARK (fwmark) support for underlay sockets (#2288)

Adds a Linux-only socket_mark u32 config flag (CLI: --socket-mark, env:
ET_SOCKET_MARK, TOML/proto: flags.socket_mark, 0 = disabled) that is
applied as SO_MARK to every outbound underlay socket EasyTier creates:
TCP, UDP, QUIC, WebSocket, WireGuard connectors and listeners, plus the
FakeTCP decoy socket. Lets the host policy-route or filter EasyTier
underlay traffic with 'ip rule fwmark ...' or iptables -m mark.

Plumbing mirrors the existing bind_device pattern:
- FlagsInConfig.socket_mark (proto) + default 0 in gen_default_flags
- bind() builder gets a socket_mark arg; setup_socket2_ext calls
  apply_socket_mark which is a no-op for mark=0 and on non-Linux
- TunnelConnector trait gets set_socket_mark(u32) default-no-op method
- IP-based connectors override; create_listener_by_url and the connector
  factory pass mark from global_ctx flags
- QUIC threads mark through QuicEndpointManager::{server,connect}
- WebSocket/FakeTCP/TCP default-bind bypass paths apply mark via
  socket2::SockRef::from(&tokio_socket)
- ForeignNetworkEntry propagates parent socket_mark into its derived ctx

Includes a Linux smoke test plus a CAP_NET_ADMIN-gated test that does a
getsockopt(SO_MARK) round-trip to confirm the kernel applied the value.

SO_MARK requires CAP_NET_ADMIN; ignored silently on non-Linux. FakeTCP's
TUN-written segments are not covered (kernel doesn't tag raw TUN
writes); operators relying on fwmark for FakeTCP must apply an iptables
rule on the FakeTCP TUN device separately.

Co-authored-by: Claude <noreply@anthropic.com>

easytier/locales/app.yml

@@ -205,6 +205,9 @@ core_clap:
   bind_device:
     en: "bind the connector socket to physical devices to avoid routing issues. e.g.: subnet proxy segment conflicts with a node's segment, after binding the physical device, it can communicate with the node normally."
     zh-CN: "将连接器的套接字绑定到物理设备以避免路由问题。比如子网代理网段与某节点的网段冲突，绑定物理设备后可以与该节点正常通信。"
+  socket_mark:
+    en: "Linux only: set SO_MARK (fwmark) on EasyTier's underlay sockets (TCP, UDP, QUIC, WebSocket, WireGuard, and the FakeTCP decoy socket) so the host can policy-route or filter them with 'ip rule fwmark ...', nftables ('meta mark'), or iptables ('-m mark'). Any value is applied verbatim (0 is a valid mark); omit the flag to leave SO_MARK untouched. Requires CAP_NET_ADMIN. Note: FakeTCP payload travels via raw TUN writes which the kernel does not tag — mark those separately on the TUN device if needed."
+    zh-CN: "仅 Linux: 在 EasyTier 的底层套接字 (TCP、UDP、QUIC、WebSocket、WireGuard 以及 FakeTCP 诱饵套接字) 上设置 SO_MARK (fwmark)，使主机能用 'ip rule fwmark ...'、nftables ('meta mark') 或 iptables ('-m mark') 策略路由/过滤这些数据包。任何值都会原样应用 (0 也是合法的 mark)；不传该参数即保持 SO_MARK 不变。需要 CAP_NET_ADMIN 权限。注意：FakeTCP 的实际载荷通过原始 TUN 写入，内核不会为其打标记；如有需要请在 TUN 设备上单独打标记。"
   enable_kcp_proxy:
     en: "proxy tcp streams with kcp, improving the latency and throughput on the network with udp packet loss."
     zh-CN: "使用 KCP 代理 TCP 流，提高在 UDP 丢包网络上的延迟和吞吐量。"