feat(icom): phase 5 — the voice keyer plays to the radio over the LAN

The audio session opens its TX side alongside RX (txenable in the conninfo,
16 kHz both ways), the stream gains a paced sender — 320 samples every 20 ms,
the frame mirroring what the rig itself sends on this socket, IDs swapped —
and the network Icom takes the same NetworkPlayer slot a TCI radio does: pick
'Radio (network audio)' as the To-radio device and the voice keyer needs no
cable and no virtual sound card. PTT brackets the message through the same
code as every other audio path.

internal/cat/icomserial.go

@@ -2047,3 +2047,20 @@ func agcValue(name string) byte {
 	}
 	return 0
 }
+
+// PlayTXAudio hands a voice-keyer message to the network audio session, when
+// this rig is reached over one. A USB-connected Icom takes its audio through
+// its own sound card instead, and says so.
+func (b *IcomSerial) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	// port is written once per Connect, on the CAT goroutine this is fetched
+	// from (IcomDo); at worst a reconnect leaves this one connection stale, and
+	// a stale transport fails fast on its closed done channel.
+	port := b.port
+	type txPlayer interface {
+		PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error
+	}
+	if p, ok := port.(txPlayer); ok {
+		return p.PlayTXAudio(pcm, rate, ch, bits, stop)
+	}
+	return fmt.Errorf("this rig takes transmit audio through its USB sound card, not the CAT link")
+}