fix: honor stereoEnabled in offline MPX path

internal/offline/generator.go

@@ -142,6 +142,10 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		// Stereo encode (produces mono, DSB-SC stereo, pilot)
 		preFrame := audio.NewFrame(audio.Sample(inL), audio.Sample(inR))
 		comps := stereoEncoder.Encode(preFrame)
+		if !g.cfg.FM.StereoEnabled {
+			comps.Stereo = 0
+			comps.Pilot = 0
+		}
 
 		// RDS
 		rdsValue := 0.0

internal/offline/generator_test.go

@@ -52,6 +52,33 @@ func TestGenerateFrameCompositeOnly(t *testing.T) {
 	}
 }
 
+func TestStereoDisabledSuppressesPilotAndStereoDifference(t *testing.T) {
+	cfgStereo := cfgpkg.Default()
+	cfgStereo.FM.FMModulationEnabled = false
+	cfgStereo.FM.StereoEnabled = true
+	cfgStereo.Audio.ToneLeftHz = 1000
+	cfgStereo.Audio.ToneRightHz = 1600
+
+	cfgMono := cfgStereo
+	cfgMono.FM.StereoEnabled = false
+
+	stereoFrame := NewGenerator(cfgStereo).GenerateFrame(20 * time.Millisecond)
+	monoFrame := NewGenerator(cfgMono).GenerateFrame(20 * time.Millisecond)
+
+	if len(stereoFrame.Samples) != len(monoFrame.Samples) {
+		t.Fatal("frame length mismatch")
+	}
+
+	var diffEnergy float64
+	for i := range stereoFrame.Samples {
+		d := float64(stereoFrame.Samples[i].I - monoFrame.Samples[i].I)
+		diffEnergy += d * d
+	}
+	if diffEnergy == 0 {
+		t.Fatal("expected stereo-enabled and stereo-disabled composite output to differ")
+	}
+}
+
 func TestWriteFile(t *testing.T) {
 	cfg := cfgpkg.Default()
 	out := filepath.Join(t.TempDir(), "test.iqf32")