fix(handler/socks5): accumulate UDP traffic metrics into recorder (issue #45)

The SOCKS5 handler recorder only tracked TCP traffic metrics. UDP
associations (CmdUdp) overwrote TCP byte counts with UDP counts, and
UDP tunnels (CmdUDPTun) recorded zero bytes.

Changes:
- udp.go: change UDP stats assignment (=) to accumulation (+=) so the
  TCP control connection bytes tracked by Handle() are preserved.
- udp_tun.go: add xstats.Stats{} wrapper around the PacketConn with
  deferred accumulation into HandlerRecorderObject, mirroring the
  handleUDP pattern.

handler/socks/v5/udp.go

@@ -97,8 +97,8 @@ func (h *socks5Handler) handleUDP(ctx context.Context, conn net.Conn, network st
 		cc = stats_wrapper.WrapPacketConn(cc, &pStats)
 
 		defer func() {
-			ro.InputBytes = pStats.Get(stats.KindInputBytes)
-			ro.OutputBytes = pStats.Get(stats.KindOutputBytes)
+			ro.InputBytes += pStats.Get(stats.KindInputBytes)
+			ro.OutputBytes += pStats.Get(stats.KindOutputBytes)
 		}()
 
 		clientID := ctxvalue.ClientIDFromContext(ctx)