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/launcher.rs

@@ -768,6 +768,10 @@ impl NetworkConfig {
             flags.bind_device = bind_device;
         }
 
+        if self.socket_mark.is_some() {
+            flags.socket_mark = self.socket_mark;
+        }
+
         if let Some(no_tun) = self.no_tun {
             flags.no_tun = no_tun;
         }
@@ -988,6 +992,7 @@ impl NetworkConfig {
         result.p2p_only = Some(flags.p2p_only);
         result.lazy_p2p = Some(flags.lazy_p2p);
         result.bind_device = Some(flags.bind_device);
+        result.socket_mark = flags.socket_mark;
         result.no_tun = Some(flags.no_tun);
         result.enable_exit_node = Some(flags.enable_exit_node);
         result.relay_all_peer_rpc = Some(flags.relay_all_peer_rpc);