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/src/tunnel/wireguard.rs

@@ -463,6 +463,7 @@ pub struct WgTunnelListener {
     wg_peer_map: Arc<DashMap<SocketAddr, Arc<WgPeer>>>,
 
     tasks: JoinSet<()>,
+    socket_mark: Option<u32>,
 }
 
 impl WgTunnelListener {
@@ -479,9 +480,14 @@ impl WgTunnelListener {
             wg_peer_map: Arc::new(DashMap::new()),
 
             tasks: JoinSet::new(),
+            socket_mark: None,
         }
     }
 
+    pub fn set_socket_mark(&mut self, socket_mark: Option<u32>) {
+        self.socket_mark = socket_mark;
+    }
+
     fn get_udp_socket(&self) -> Arc<UdpSocket> {
         self.udp.as_ref().unwrap().clone()
     }
@@ -561,6 +567,7 @@ impl TunnelListener for WgTunnelListener {
                 .addr(addr)
                 .only_v6(true)
                 .maybe_dev(tunnel_url.bind_dev())
+                .maybe_socket_mark(self.socket_mark)
                 .call()?,
         ));
         self.addr
@@ -599,6 +606,7 @@ pub struct WgTunnelConnector {
     bind_addrs: Vec<SocketAddr>,
     ip_version: IpVersion,
     resolved_addr: Option<SocketAddr>,
+    socket_mark: Option<u32>,
 }
 
 impl Debug for WgTunnelConnector {
@@ -619,6 +627,7 @@ impl WgTunnelConnector {
             bind_addrs: vec![],
             ip_version: IpVersion::Both,
             resolved_addr: None,
+            socket_mark: None,
         }
     }
 
@@ -695,6 +704,7 @@ impl WgTunnelConnector {
             .addr("[::]:0".parse().unwrap())
             .dev(BindDev::Disabled)
             .only_v6(true)
+            .maybe_socket_mark(self.socket_mark)
             .call()?;
         Self::connect_with_socket(self.addr.clone(), self.config.clone(), socket, addr).await
     }
@@ -721,7 +731,12 @@ impl super::TunnelConnector for WgTunnelConnector {
         let futures = FuturesUnordered::new();
         for bind_addr in bind_addrs.into_iter() {
             tracing::info!(?bind_addr, ?addr, "bind addr");
-            match bind().addr(bind_addr).only_v6(true).call() {
+            match bind()
+                .addr(bind_addr)
+                .only_v6(true)
+                .maybe_socket_mark(self.socket_mark)
+                .call()
+            {
                 Ok(socket) => futures.push(Self::connect_with_socket(
                     self.addr.clone(),
                     self.config.clone(),
@@ -753,6 +768,10 @@ impl super::TunnelConnector for WgTunnelConnector {
     fn set_resolved_addr(&mut self, addr: SocketAddr) {
         self.resolved_addr = Some(addr);
     }
+
+    fn set_socket_mark(&mut self, socket_mark: Option<u32>) {
+        self.socket_mark = socket_mark;
+    }
 }
 
 #[cfg(test)]