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/icomaudio.go

@@ -22,6 +22,7 @@ package cat
 // audio stream is opt-in and entirely separate from control/CI-V.
 
 import (
+	"fmt"
 	"net"
 	"sync"
 	"sync/atomic"
@@ -236,3 +237,58 @@ func (a *icomAudio) sendRetransmitReq() {
 		_, _ = a.conn.Write(b)
 	}
 }
+
+// PlayTX sends one already-decoded message out over the audio session, paced
+// at the stream's own 20 ms / 320-sample cadence, and returns when the last
+// packet has gone or stop is closed.
+//
+// The frame mirrors what the rig itself sends on this socket byte for byte —
+// ident 0x81 0x01 (LPCM mono 16-bit), a big-endian send sequence at 0x12, the
+// payload length at 0x14, PCM from 0x18 — with the IDs swapped for direction.
+// PTT is the caller's business, exactly as on the TCI and sound-card paths.
+func (a *icomAudio) PlayTX(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error {
+	mono := decodeToMono(pcm, ch, bits)
+	if len(mono) == 0 {
+		return fmt.Errorf("the message is empty")
+	}
+	if rate > 0 && rate != 16000 {
+		mono = resampleLinear(mono, rate, 16000)
+	}
+	const frame = 320 // samples per packet: 20 ms at 16 kHz, the rig's own cadence
+	tick := time.NewTicker(20 * time.Millisecond)
+	defer tick.Stop()
+	var outerSeq uint16 = 1
+	var sendSeq uint16
+	for pos := 0; pos < len(mono); pos += frame {
+		select {
+		case <-stop:
+			return nil
+		case <-a.done:
+			return fmt.Errorf("the audio stream closed mid-message")
+		case <-tick.C:
+		}
+		pkt := make([]byte, 0x18+frame*2)
+		icnLE.PutUint32(pkt[0:], uint32(len(pkt)))
+		icnLE.PutUint16(pkt[6:], outerSeq)
+		icnLE.PutUint32(pkt[8:], a.aID)
+		icnLE.PutUint32(pkt[12:], a.aRemote)
+		pkt[0x10], pkt[0x11] = 0x81, 0x01
+		icnBE.PutUint16(pkt[0x12:], sendSeq)
+		icnBE.PutUint32(pkt[0x14:], frame*2)
+		for i := 0; i < frame && pos+i < len(mono); i++ {
+			v := mono[pos+i] * 32767
+			if v > 32767 {
+				v = 32767
+			} else if v < -32768 {
+				v = -32768
+			}
+			icnLE.PutUint16(pkt[0x18+i*2:], uint16(int16(v)))
+		}
+		if _, err := a.conn.Write(pkt); err != nil {
+			return fmt.Errorf("sending audio to the rig: %w", err)
+		}
+		outerSeq++
+		sendSeq++
+	}
+	return nil
+}