fix(wrapper): prevent nil-pointer dereference in WrapUDPConn chain when stats is nil

stats.WrapUDPConn returned nil when pStats was nil (introduced in 7c95d40),
but callers in the kcp and rudp listeners passed the result directly to
admission.WrapUDPConn and limiter.WrapUDPConn, which wrapped a nil PacketConn.
When service.Serve() called conn.LocalAddr(), the auto-generated method
delegated to the nil PacketConn, causing a panic.

Root cause: WrapUDPConn behavior was inconsistent with WrapConn and
WrapPacketConn in the same package, which both return the original connection
instead of nil when the optional config is absent.

Fixes:

1. listener/kcp, listener/rudp — guard stats.WrapUDPConn with Stats != nil
   (consistent with quic and wt listeners which already had this guard).

2. observer/stats/wrapper — when pStats is nil, pass through the original
   connection instead of returning nil (matches WrapConn/WrapPacketConn).

3. admission/wrapper, limiter/traffic/wrapper — add defensive nil guards:
   nil pc → return nil; nil config → pass through original connection.
   All four WrapUDPConn implementations now share the same nil semantics.

4. listener/udp_wrapper_safety_test.go — regression test that verifies the
   full metrics→stats→admission→limiter chain with all optional configs nil
   does not produce a wrapper with a nil internal PacketConn.

Fixes: go-gost/gost#873

observer/stats/wrapper/conn.go

@@ -156,10 +156,18 @@ type udpConn struct {
 // accepted UDP client session == one connection, mirroring WrapConn) and
 // tracks bytes read and written across all read/write methods (Read, Write,
 // ReadFrom, WriteTo, ReadFromUDP, ReadMsgUDP, WriteToUDP, WriteMsgUDP). On
-// Close, it decrements the current connection count. If pc or pStats is nil,
-// it returns nil.
+// Close, it decrements the current connection count.
+//
+// If pc is nil, nil is returned. If pStats is nil, the original connection
+// is returned unchanged (consistent with WrapConn and WrapPacketConn).
 func WrapUDPConn(pc net.PacketConn, pStats stats.Stats) udp.Conn {
-	if pc == nil || pStats == nil {
+	if pc == nil {
+		return nil
+	}
+	if pStats == nil {
+		if uc, ok := pc.(udp.Conn); ok {
+			return uc
+		}
 		return nil
 	}