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/icomnet.go

@@ -940,7 +940,11 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	copy(b[0x40:0x60], []byte("IC-7610"))
 	copy(b[0x60:0x70], icnPasscode(user))
 	b[0x70] = rxEnable // rxenable: 1 opens the 50003 RX audio stream, 0 = CI-V only
-	b[0x71] = 0x00     // txenable (Phase 5)
+	// TX rides the same audio session: whenever the operator wants RX audio the
+	// TX side is opened with it, so the voice keyer can play to the radio with
+	// no cable. Costs nothing when unused — no packets flow until a message is
+	// actually played.
+	b[0x71] = rxEnable // txenable
 	// rxcodec 0x04 = LPCM, ONE channel, 16-BIT — settled by experiment on a
 	// real IC-7760, one wrong guess at a time: 0x10 produced 1280-byte payloads
 	// of interleaved 16-bit stereo, 0x02 produced 320-byte payloads of 8-bit
@@ -949,7 +953,7 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	b[0x72] = 0x04 // rxcodec
 	b[0x73] = 0x04 // txcodec
 	icnBE.PutUint32(b[0x74:], 16000)
-	icnBE.PutUint32(b[0x78:], 8000)
+	icnBE.PutUint32(b[0x78:], 16000) // TX sample rate — the same 16 kHz everything else here runs at
 	icnBE.PutUint32(b[0x7c:], uint32(civPort))
 	icnBE.PutUint32(b[0x80:], uint32(audioPort))
 	icnBE.PutUint32(b[0x84:], 100)
@@ -957,6 +961,17 @@ func icnConnInfo(seq, innerSeq, tokReq uint16, sentid, rcvdid, token uint32, use
 	return b
 }
 
+// PlayTXAudio plays one voice-keyer message through the 50003 audio session.
+// On the transport because the transport owns the session; the IcomSerial
+// controller forwards here after a type assertion, exactly as the TCI path
+// reaches its radio.
+func (n *icomNet) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	if n.audio == nil {
+		return fmt.Errorf("the radio's audio stream is not open — enable RX audio in Settings → CAT first")
+	}
+	return n.audio.PlayTX(pcm, rate, ch, bits, stop)
+}
+
 func icnOpenClose(seq uint16, sentid, rcvdid uint32, civSeq uint16, magic byte) []byte {
 	b := make([]byte, 0x16)
 	icnLE.PutUint32(b[0:], 0x16)