New split writer + a `split` flag through the capture path: when the mic is on,
audio.wav is recorded 2ch with the mic on the left channel and the loopback
(downmixed) on the right, keeping the two sources separate in the file. Same
byte size as today's summed stereo. Transcription still downmixes L+R (= the
summed signal). Unit test covers L=mic / R=loopback.