feat(tci): the QSO recorder can take its audio from the radio

Confirmed on a real SunSDR: the stream decodes and the test recording
plays back clean. So it can do the job a virtual audio cable was doing —
this wires it to the QSO recorder, which already accepts a pushed source
(the Icom network audio uses the same door).

The conversion lives here rather than in internal/cat: the radio's job is
to hand over what it sent, not to know that the recorder works in 16 kHz
mono. Three samples are AVERAGED rather than two of them dropped —
decimating by picking every third folds everything above 8 kHz back into
the voice band, and on a receiver that is hiss, which a QSO recording has
plenty of already.

Off by default, and applied the moment it is switched: it replaces a
sound card the operator has already wired up, and an option that needs a
restart to take effect reads as an option that does not work.

internal/audio/rate.go

@@ -0,0 +1,9 @@
+package audio
+
+// RecorderSampleRate is the rate the QSO recorder works in.
+//
+// Exported because a source that is NOT a sound card — the TCI receive stream,
+// the Icom network audio — has to resample into it, and hard-coding 16000 at
+// each of those call sites is how one of them ends up at the wrong speed after
+// this constant is ever changed.
+const RecorderSampleRate = sampleRate