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/icomaudio.go

@@ -57,8 +57,14 @@ type icomAudio struct {
 	rxLastSeq uint16
 	rxMissing map[uint16]int
 
-	dumped int          // packets hex-dumped so far (≤ icaDumpFirst)
-	lastRx atomic.Int64 // UnixNano of last packet (liveness)
+	dumped int // packets hex-dumped so far (≤ icaDumpFirst)
+
+	// Live-TX state — see SendTXChunk.
+	txMu    sync.Mutex
+	txRem   []byte
+	txOuter uint16
+	txSend  uint16
+	lastRx  atomic.Int64 // UnixNano of last packet (liveness)
 
 	done      chan struct{}
 	closeOnce sync.Once
@@ -292,3 +298,38 @@ func (a *icomAudio) PlayTX(pcm []byte, rate, ch, bits int, stop <-chan struct{})
 	}
 	return nil
 }
+
+// Live TX — the microphone, not a recorded message. Chunks arrive at the
+// microphone's own real-time pace (16 kHz mono 16-bit, whatever length the
+// capture delivers) and are re-framed into the rig's 320-sample packets; the
+// remainder waits for the next chunk. Counters and remainder live on the
+// stream so a talk session survives across calls.
+func (a *icomAudio) SendTXChunk(pcm []byte) error {
+	a.txMu.Lock()
+	defer a.txMu.Unlock()
+	a.txRem = append(a.txRem, pcm...)
+	const frameBytes = 640
+	for len(a.txRem) >= frameBytes {
+		select {
+		case <-a.done:
+			return fmt.Errorf("the audio stream closed")
+		default:
+		}
+		pkt := make([]byte, 0x18+frameBytes)
+		icnLE.PutUint32(pkt[0:], uint32(len(pkt)))
+		a.txOuter++
+		icnLE.PutUint16(pkt[6:], a.txOuter)
+		icnLE.PutUint32(pkt[8:], a.aID)
+		icnLE.PutUint32(pkt[12:], a.aRemote)
+		pkt[0x10], pkt[0x11] = 0x81, 0x01
+		icnBE.PutUint16(pkt[0x12:], a.txSend)
+		a.txSend++
+		icnBE.PutUint32(pkt[0x14:], frameBytes)
+		copy(pkt[0x18:], a.txRem[:frameBytes])
+		a.txRem = a.txRem[frameBytes:]
+		if _, err := a.conn.Write(pkt); err != nil {
+			return fmt.Errorf("sending mic audio to the rig: %w", err)
+		}
+	}
+	return nil
+}