feat(tci): name the transmit audio source when keying, per the protocol

The specification settles what a night of experiments could only guess at.
TRX takes an optional THIRD argument naming the signal source — tci, mic1,
mic2, micPC, ecoder2 — and TCI 2.0 says it plainly: 'The signal for
transmitting is always taken from the microphone selected in the
ExpertSDR3. If a third-party software connected via TCI wants to transmit
its audio signal, you must specify the third argument - TCI.'

Without it the radio sends no chrono at all, whatever the mode. That is
what the SSB attempts ran into, and what was misread here as 'digital
modes only' — the mode was never the rule, the missing argument was.

So OpsLog says it, following the 'To radio' device: 'tci' when the voice
keyer owns the transmission, nothing at all otherwise, which leaves the
operator's own microphone alone for every other PTT. Nobody has to find
that setting in ExpertSDR3 and set it again for every mode, which is how
it is remembered there.

Two more things from the same document. A chrono with no audio ready is
now answered with silence rather than left unanswered — the vendor calls
that preferable. And the receive stream declares float32 and two channels
instead of trusting the defaults: they are the documented defaults, but a
default is something another program sharing this radio can have changed,
and a stream in an unexpected format is heard as noise rather than as a
mistake.

internal/cat/tci_audio.go

@@ -136,6 +136,13 @@ func (t *TCI) StartTCIAudio(rx, rate int) error {
 	if err := t.send(fmt.Sprintf("audio_samplerate:%d;", rate)); err != nil {
 		return err
 	}
+	// Said rather than assumed. float32 and two channels are the documented
+	// defaults and what this radio streams, but a default is a thing another
+	// program can have changed — they share the radio, not just the protocol —
+	// and a stream arriving in a format the decoder was not expecting is heard
+	// as noise, not as a mistake.
+	_ = t.send("audio_stream_sample_type:float32;")
+	_ = t.send("audio_stream_channels:2;")
 	return t.send(fmt.Sprintf("audio_start:%d;", rx))
 }