fix: a silent audio source no longer empties the whole recording

From an operator's log: a Flex with From Radio on DAX RX and Recording mic on
DAX Mic, a full CW QSO recorded, and at log time "snapshot failed: recording
was empty".

The mixer took min(len(A), len(B)) samples. In CW the mic path does not run, so
DAX Mic delivered nothing, min stayed 0, and the drift guard then threw the
radio audio away a second at a time. Two devices configured meant the recording
depended on the quieter one being alive.

A source that has delivered nothing for 1.5 s is now treated as absent and the
other is recorded alone, with one line in the log saying so — half a recording
is worth having, silence is not. Both are mixed again the moment the missing one
comes back.

Neither is written off before it has EVER spoken: at startup both timestamps are
zero, and declaring one dead then would clip the opening of every recording.
That case is tested too.

internal/audio/recorder_deadsource_test.go

@@ -0,0 +1,67 @@
+package audio
+
+import (
+	"testing"
+	"time"
+)
+
+// A silent second source must not silence the recording.
+//
+// From a real session: a Flex with "From Radio" on DAX RX and "Recording mic"
+// on DAX Mic. In CW the mic path does not run, so DAX Mic delivered nothing —
+// and because the mixer took min(len(A), len(B)) samples, nothing at all was
+// recorded. The whole QSO ended in "recording was empty".
+func TestMixerSurvivesASilentSource(t *testing.T) {
+	r := &Recorder{twoSrc: true, gainA: 1, gainB: 1, running: true, active: true, prerollSamples: sampleRate}
+
+	// The radio has been talking; the mic has not spoken for well over the
+	// grace period.
+	r.bufA = make([]int16, 800)
+	for i := range r.bufA {
+		r.bufA[i] = 1000
+	}
+	r.lastA = time.Now()
+	r.lastB = time.Now().Add(-5 * time.Second)
+
+	r.mixTick()
+
+	if len(r.acc) == 0 {
+		t.Fatal("nothing was recorded — a silent mic must not stop the radio being captured")
+	}
+	if r.acc[0] != 1000 {
+		t.Errorf("sample = %d, want 1000 — the live source must pass through unchanged", r.acc[0])
+	}
+}
+
+// While BOTH sources are alive the two are mixed, as before.
+func TestMixerStillMixesBothSources(t *testing.T) {
+	r := &Recorder{twoSrc: true, gainA: 1, gainB: 1, running: true, active: true, prerollSamples: sampleRate}
+	r.bufA = []int16{100, 100, 100}
+	r.bufB = []int16{50, 50, 50}
+	now := time.Now()
+	r.lastA, r.lastB = now, now
+
+	r.mixTick()
+
+	if len(r.acc) != 3 {
+		t.Fatalf("recorded %d samples, want 3", len(r.acc))
+	}
+	if r.acc[0] != 150 {
+		t.Errorf("sample = %d, want 150 — both sources should be summed", r.acc[0])
+	}
+}
+
+// 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) {
+	r := &Recorder{twoSrc: true, gainA: 1, gainB: 1, running: true, active: true, prerollSamples: sampleRate}
+	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))
+	}
+}