fix: keep the audio captured while waiting for a silent source

The operator asked the right question: why wait 1.5 s at all, when the mode is
known from CAT?

Because the wait is not the problem — what happened during it was. The drift
guard, meant for two sound cards running on their own clocks, saw a starved
source as a one-second skew and trimmed the radio audio to match. So the opening
of every CW recording was thrown away although it had been captured.

The guard now applies only while BOTH sources are actually delivering. Waiting
then costs nothing: everything buffered is written whole the moment the silent
source is written off, which a test now pins with three seconds of audio.

Using the CAT mode to skip the wait would have been a second mechanism for a
case this one already covers — and it would not cover the others: a device
switched off, unplugged, or held by another program.

internal/audio/recorder.go

@@ -335,12 +335,20 @@ func (r *Recorder) mixTick() {
 			r.bufA = append(r.bufA[:0], r.bufA[n:]...)
 			r.bufB = append(r.bufB[:0], r.bufB[n:]...)
 		}
-		// Drift guard: if the clocks diverge, drop the excess so the two
-		// sources stay roughly aligned (≤1 s skew).
-		if d := len(r.bufA) - len(r.bufB); d > sampleRate {
-			r.bufA = append(r.bufA[:0], r.bufA[d:]...)
-		} else if d < -sampleRate {
-			r.bufB = append(r.bufB[:0], r.bufB[-d:]...)
+		// Drift guard: two sound cards run on their own clocks, so drop the
+		// excess to keep them within a second of each other.
+		//
+		// ONLY while both are actually running. A starved source is not drift: it
+		// made this guard throw away the radio audio a second at a time during
+		// the grace period, so the opening of every recording was lost even
+		// though it had been captured. Waiting costs nothing now — whatever is
+		// buffered is written whole the moment the silent source is written off.
+		if len(r.bufA) > 0 && len(r.bufB) > 0 {
+			if d := len(r.bufA) - len(r.bufB); d > sampleRate {
+				r.bufA = append(r.bufA[:0], r.bufA[d:]...)
+			} else if d < -sampleRate {
+				r.bufB = append(r.bufB[:0], r.bufB[-d:]...)
+			}
 		}
 	} else if len(r.bufA) > 0 {
 		mixed = make([]int16, len(r.bufA))