fix: write off an audio source that never delivers anything

The operator said it from the start: in CW there is no sidetone, only reception.
My previous fix did not cover it — it wrote off a source that STOPPED, but a
source that never starts has no last-delivery time, so it stayed forever "not
yet dry" and held the mixer at min(A,B)=0. Three empty recordings while the
radio audio streamed perfectly.

A source with no delivery is now measured from when capture started. Tested both
ways: nothing is written off in the first moments, and a source that has never
spoken is written off once the grace period has passed.

internal/audio/recorder_deadsource_test.go

@@ -51,17 +51,40 @@ func TestMixerStillMixesBothSources(t *testing.T) {
 	}
 }
 
-// Before either source has EVER delivered, neither may be written off: at
-// startup both timestamps are zero, and declaring one dead then would drop the
-// first moments of every recording.
-func TestMixerWaitsForBothAtStartup(t *testing.T) {
+// Just after capture starts, a source that has not delivered yet is NOT written
+// off: the first samples take a moment to arrive, and declaring the mic dead at
+// once would drop the opening of every recording.
+func TestMixerWaitsBrieflyAtStartup(t *testing.T) {
 	r := &Recorder{twoSrc: true, gainA: 1, gainB: 1, running: true, active: true, prerollSamples: sampleRate}
+	r.startedAt = time.Now()
 	r.bufA = []int16{100, 100}
-	// lastA/lastB both zero — nothing has been heard from either yet.
 
 	r.mixTick()
 
 	if len(r.acc) != 0 {
-		t.Errorf("recorded %d samples before the second source was known to be silent", len(r.acc))
+		t.Errorf("recorded %d samples immediately — the second source deserves a moment to start", len(r.acc))
+	}
+}
+
+// A source that has NEVER delivered is written off once enough time has passed.
+//
+// This is the case that actually happens, and the one the first version missed:
+// a DAX Mic channel switched off delivers not one sample, so it had no
+// last-delivery time and stayed forever "not yet dry" — taking the whole
+// recording down with it while the radio audio streamed perfectly.
+func TestMixerWritesOffASourceThatNeverSpoke(t *testing.T) {
+	r := &Recorder{twoSrc: true, gainA: 1, gainB: 1, running: true, active: true, prerollSamples: sampleRate}
+	r.startedAt = time.Now().Add(-5 * time.Second)
+	r.bufA = []int16{700, 700, 700}
+	r.lastA = time.Now()
+	// lastB stays zero: the mic has never produced anything at all.
+
+	r.mixTick()
+
+	if len(r.acc) != 3 {
+		t.Fatalf("recorded %d samples, want 3 — the radio was streaming the whole time", len(r.acc))
+	}
+	if r.acc[0] != 700 {
+		t.Errorf("sample = %d, want 700", r.acc[0])
 	}
 }