test(stereo): cover mono delay compensation

Add regression coverage for SSB/VSB mono delay compensation, confirm DSB mono remains immediate, and verify Reset clears the delay state before reuse.

internal/stereo/delay_test.go

@@ -0,0 +1,60 @@
+package stereo
+
+import (
+	"math"
+	"testing"
+
+	"github.com/jan/fm-rds-tx/internal/audio"
+)
+
+func TestSSBAndVSBDelayMonoToMatchDiffPath(t *testing.T) {
+	const sampleRate = 228000
+	const wantDelay = (ssbHilbertTaps - 1) / 2
+
+	for _, mode := range []Mode{ModeSSB, ModeVSB} {
+		enc := NewStereoEncoder(sampleRate)
+		enc.SetMode(mode, sampleRate)
+
+		for i := 0; i < wantDelay; i++ {
+			c := enc.Encode(audio.NewFrame(1, 1))
+			if math.Abs(c.Mono) > 1e-12 {
+				t.Fatalf("mode %s: mono leaked before delay filled at sample %d: got %.9f want 0", mode, i, c.Mono)
+			}
+		}
+
+		c := enc.Encode(audio.NewFrame(1, 1))
+		if math.Abs(c.Mono-1) > 1e-12 {
+			t.Fatalf("mode %s: mono did not emerge after expected delay: got %.9f want 1", mode, c.Mono)
+		}
+	}
+}
+
+func TestDSBMonoRemainsUndelayed(t *testing.T) {
+	enc := NewStereoEncoder(228000)
+	enc.SetMode(ModeDSB, 228000)
+	c := enc.Encode(audio.NewFrame(1, 1))
+	if math.Abs(c.Mono-1) > 1e-12 {
+		t.Fatalf("DSB mono should be immediate: got %.9f want 1", c.Mono)
+	}
+}
+
+func TestResetClearsMonoDelayState(t *testing.T) {
+	const sampleRate = 228000
+	const wantDelay = (ssbHilbertTaps - 1) / 2
+
+	enc := NewStereoEncoder(sampleRate)
+	enc.SetMode(ModeSSB, sampleRate)
+
+	for i := 0; i < wantDelay+4; i++ {
+		_ = enc.Encode(audio.NewFrame(1, 1))
+	}
+
+	enc.Reset()
+
+	for i := 0; i < wantDelay; i++ {
+		c := enc.Encode(audio.NewFrame(1, 1))
+		if math.Abs(c.Mono) > 1e-12 {
+			t.Fatalf("reset did not clear mono delay at sample %d: got %.9f want 0", i, c.Mono)
+		}
+	}
+}