fix(observer): remove dead nil checks from udpConn, add unit tests

The WrapUDPConn constructor returns nil when either arg is nil, so
c.stats is always non-nil in all udpConn methods. Remove 8 redundant
nil guards that were inconsistent with packetConn (which already
skipped them).

Add 65 unit tests across plugin, stats, and wrapper packages covering
nil safety, race conditions, double-close, event marshaling, and
reset-traffic semantics. Replace magic numbers in tests with named
Kind constants.

observer/stats/wrapper/io_test.go

@@ -0,0 +1,90 @@
+package wrapper
+
+import (
+	"testing"
+
+	"github.com/go-gost/core/observer/stats"
+	ostats "github.com/go-gost/x/observer/stats"
+)
+
+type fakeReadWriter struct {
+	readBuf  []byte
+	readPos  int
+	writeBuf []byte
+}
+
+func (rw *fakeReadWriter) Read(b []byte) (int, error) {
+	if rw.readPos >= len(rw.readBuf) {
+		return 0, nil
+	}
+	n := copy(b, rw.readBuf[rw.readPos:])
+	rw.readPos += n
+	return n, nil
+}
+
+func (rw *fakeReadWriter) Write(b []byte) (int, error) {
+	rw.writeBuf = append(rw.writeBuf, b...)
+	return len(b), nil
+}
+
+func TestWrapReadWriter_Nil(t *testing.T) {
+	// nil rw
+	if result := WrapReadWriter(nil, ostats.NewStats(false)); result != nil {
+		t.Error("WrapReadWriter with nil rw should return nil")
+	}
+	// nil stats
+	frw := &fakeReadWriter{}
+	if result := WrapReadWriter(frw, nil); result != frw {
+		t.Error("WrapReadWriter with nil stats should return original rw")
+	}
+	// both nil
+	if result := WrapReadWriter(nil, nil); result != nil {
+		t.Error("WrapReadWriter with both nil should return nil")
+	}
+}
+
+func TestWrapReadWriter_Read(t *testing.T) {
+	st := ostats.NewStats(false)
+	frw := &fakeReadWriter{readBuf: []byte("testdata")}
+	wrapped := WrapReadWriter(frw, st)
+
+	buf := make([]byte, 8)
+	n, _ := wrapped.Read(buf)
+	if n != 8 {
+		t.Fatalf("read n = %d, want 8", n)
+	}
+	if st.Get(stats.KindInputBytes) != 8 {
+		t.Errorf("inputBytes = %d, want 8", st.Get(stats.KindInputBytes))
+	}
+}
+
+func TestWrapReadWriter_Write(t *testing.T) {
+	st := ostats.NewStats(false)
+	frw := &fakeReadWriter{}
+	wrapped := WrapReadWriter(frw, st)
+
+	n, _ := wrapped.Write([]byte("hello"))
+	if n != 5 {
+		t.Fatalf("write n = %d, want 5", n)
+	}
+	if st.Get(stats.KindOutputBytes) != 5 {
+		t.Errorf("outputBytes = %d, want 5", st.Get(stats.KindOutputBytes))
+	}
+}
+
+func TestWrapReadWriter_ReadWrite(t *testing.T) {
+	st := ostats.NewStats(false)
+	frw := &fakeReadWriter{readBuf: []byte("abcdefghij")}
+	wrapped := WrapReadWriter(frw, st)
+
+	buf := make([]byte, 5)
+	wrapped.Read(buf)  // reads 5 bytes
+	wrapped.Write([]byte("xyz")) // writes 3 bytes
+
+	if st.Get(stats.KindInputBytes) != 5 {
+		t.Errorf("inputBytes = %d, want 5", st.Get(stats.KindInputBytes))
+	}
+	if st.Get(stats.KindOutputBytes) != 3 {
+		t.Errorf("outputBytes = %d, want 3", st.Get(stats.KindOutputBytes))
+	}
+}