Pause playback while transmitting and declare microphone use.

macOS refuses capture without a usage string, and queued remote audio was still handed to the speaker after push-to-talk began.

Co-authored-by: Cursor <cursoragent@cursor.com>

station/behavior_test.go

@@ -360,6 +360,32 @@ func TestRemoteAudioAndTalker(t *testing.T) {
 	}
 }
 
+func TestPlaybackPausesWhenTransmitting(t *testing.T) {
+	in := login(t, "DL1ABC", "key", 20, 7, []string{"OPUS"})
+	enc, err := opus.NewEncoder()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer enc.Close()
+	pkt, err := enc.Encode(make([]int16, opus.FrameSamples))
+	if err != nil {
+		t.Fatal(err)
+	}
+	in.refl.writeUDP(t, 101, 7, 1, appendBytes(nil, pkt))
+	in.refl.writeUDP(t, 101, 7, 2, appendBytes(nil, pkt))
+	if _, err := in.st.WaitPlayback(t.Context(), 2*opus.FrameSamples-1); err != nil {
+		t.Fatal(err)
+	}
+	in.st.HoldButton()
+	in.refl.writeFrame(t, 105, appendStr(appendU32(nil, 20), "DB0XYZ"))
+	if _, err := in.st.Wait(t.Context(), func(s station.State) bool { return s.Transmitting }); err != nil {
+		t.Fatal(err)
+	}
+	if pcm := in.st.TakePlayback(); len(pcm) != 0 {
+		t.Fatalf("playback %d samples, want none while transmitting", len(pcm))
+	}
+}
+
 func TestMicrophoneIsNotPlayed(t *testing.T) {
 	in := login(t, "DL1ABC", "key", 20, 7, []string{"OPUS"})
 	in.st.HoldButton()