feat(icom): live microphone over the network — the last remote brick

The talk button learns the network road the voice keyer already took: when
To-radio is the radio itself, the microphone is captured and re-framed into
the rig's 320-sample packets, the capture callback serving as the clock — the
mic delivers in real time, so no ring and no pacer. Counters and the frame
remainder live on the audio stream, so a talk session spans calls. PTT keys
before and releases after, through the same code as the USB path.

With this, a network Icom is a complete remote station over three UDP ports:
RX audio to the headset, live voice and recorded messages back, CW through
the rig's keyer, CAT for everything else.

internal/cat/icomnet.go

@@ -972,6 +972,15 @@ func (n *icomNet) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct
 	return n.audio.PlayTX(pcm, rate, ch, bits, stop)
 }
 
+// TXAudioSender returns a function that streams live microphone chunks to the
+// rig — the talk button's road, where PlayTXAudio is the voice keyer's.
+func (n *icomNet) TXAudioSender() (func([]byte) error, error) {
+	if n.audio == nil {
+		return nil, fmt.Errorf("the radio's audio stream is not open — enable RX audio in Settings → CAT first")
+	}
+	return n.audio.SendTXChunk, nil
+}
+
 func icnOpenClose(seq uint16, sentid, rcvdid uint32, civSeq uint16, magic byte) []byte {
 	b := make([]byte, 0x16)
 	icnLE.PutUint32(b[0:], 0x16)