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/proto/api_manage.proto

@@ -101,6 +101,7 @@ message NetworkConfig {
   optional string ipv6_public_addr_prefix = 64;
   optional bool disable_relay_data = 65;
   optional bool enable_udp_broadcast_relay = 66;
+  optional uint32 socket_mark = 67;
 }
 
 message PortForwardConfig {

easytier/src/proto/common.proto

@@ -77,6 +77,13 @@ message FlagsInConfig {
   bool disable_upnp = 40;
   bool disable_relay_data = 41;
   bool enable_udp_broadcast_relay = 42;
+
+  // Linux-only: SO_MARK (fwmark) value applied to every outbound underlay
+  // socket (TCP/UDP/QUIC/WS/WG connectors and listeners). Unset = leave
+  // SO_MARK untouched (kernel default 0). Any set value (including 0) is
+  // applied via setsockopt. Requires CAP_NET_ADMIN; silently ignored on
+  // non-Linux platforms.
+  optional uint32 socket_mark = 43;
 }
 
 message RpcDescriptor {