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.

changelog.json

@@ -6,13 +6,15 @@
       "FlexRadio: in split, OpsLog stays on the receive slice instead of following the transmitter.",
       "QSO recording works on CW again. Your own sidetone is not in the file, the other station is.",
       "Playing a recording on the air now says why when it cannot — most often no audio output to the radio is configured.",
-      "The play-on-air button is hidden on CW: the transmitter builds its tone from the keyer and ignores the audio path. Recording, stopping and resuming stay available."
+      "The play-on-air button is hidden on CW: the transmitter builds its tone from the keyer and ignores the audio path. Recording, stopping and resuming stay available.",
+      "A QSO recording no longer comes out empty when one of the two audio sources is silent — the case in CW, where the mic device delivers nothing. The live source is recorded alone."
     ],
     "fr": [
       "FlexRadio : en split, OpsLog reste sur la slice de réception au lieu de suivre l'émission.",
       "L'enregistrement des QSO fonctionne à nouveau en CW. Votre propre signal d'écoute n'est pas dans le fichier, celui du correspondant l'est.",
       "L'émission d'un enregistrement indique désormais pourquoi elle échoue — le plus souvent aucune sortie audio vers la radio n'est configurée.",
-      "Le bouton d'émission de l'enregistrement est masqué en CW : l'émetteur fabrique sa tonalité à partir du manipulateur et ignore la chaîne audio. Enregistrer, arrêter et reprendre restent disponibles."
+      "Le bouton d'émission de l'enregistrement est masqué en CW : l'émetteur fabrique sa tonalité à partir du manipulateur et ignore la chaîne audio. Enregistrer, arrêter et reprendre restent disponibles.",
+      "Un enregistrement de QSO n'est plus vide quand l'une des deux sources audio est muette — le cas en CW, où le périphérique micro ne débite rien. La source active est enregistrée seule."
     ]
   },
   {