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/icomserial.go

@@ -2064,3 +2064,16 @@ func (b *IcomSerial) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan str
 	}
 	return fmt.Errorf("this rig takes transmit audio through its USB sound card, not the CAT link")
 }
+
+// TXAudioSender hands back the network transport's live-mic sender, when this
+// rig is reached over one.
+func (b *IcomSerial) TXAudioSender() (func([]byte) error, error) {
+	port := b.port
+	type sender interface {
+		TXAudioSender() (func([]byte) error, error)
+	}
+	if p, ok := port.(sender); ok {
+		return p.TXAudioSender()
+	}
+	return nil, fmt.Errorf("this rig takes transmit audio through its USB sound card, not the CAT link")
+}