fix(icom): only forward-moving audio packets reach the sink

Every audio packet was delivered as if it were the next 20 ms, duplicates and
late retransmits included. The monitor's capped ring quietly threw the surplus
away, so the speakers sounded fine — but the recorder keeps every sample it is
given, and the file came out longer than the QSO, slowed and stuttering, each
lost-then-resent packet heard twice. trackRxSeq now says whether a packet
advances the stream, and only those are handed on.

internal/cat/icomaudio.go

@@ -134,12 +134,19 @@ func (a *icomAudio) audioPump() {
 			case typ == 0x05: // rig-initiated disconnect
 				debugLog.Printf("icom audio: rig sent DISCONNECT — audio stream dropped by the rig")
 			case typ == 0x00 && k > icaAudioOffset: // audio data packet
-				a.trackRxSeq(icnLE.Uint16(buf[6:]))
+				fresh := a.trackRxSeq(icnLE.Uint16(buf[6:]))
 				if a.dumped < icaDumpFirst {
 					a.dumped++
 					debugLog.Printf("icom audio raw #%d: len=%d head=% X", a.dumped, k, buf[:min(icaAudioOffset+8, k)])
 				}
-				if a.sink != nil {
+				// Only a packet that ADVANCES the sequence reaches the sink. A
+				// duplicate or a late retransmit used to be delivered as if it
+				// were the next 20 ms of audio: the monitor's capped ring threw
+				// the surplus away (the speakers stayed clean), but the recorder
+				// keeps every sample it is given — the file grew longer than the
+				// QSO and played back slowed and stuttering, each lost-then-
+				// resent packet heard twice.
+				if fresh && a.sink != nil {
 					payload := append([]byte(nil), buf[icaAudioOffset:k]...)
 					a.sink(payload)
 				}
@@ -159,26 +166,36 @@ func (a *icomAudio) audioPump() {
 // trackRxSeq / sendRetransmitReq mirror icomNet's receive-side retransmit exactly
 // (audio is as loss-sensitive as the scope stream). Duplicated deliberately so
 // the audio stream owns its own seq state with no shared locking.
-func (a *icomAudio) trackRxSeq(seq uint16) {
+//
+// The return value says whether this packet moves the stream FORWARD — the
+// only kind the sink may hear. A duplicate is the same 20 ms again; a late
+// retransmit would play old audio in the middle of new. Both are accounted
+// for here and dropped by the caller.
+func (a *icomAudio) trackRxSeq(seq uint16) bool {
 	if !a.rxHaveSeq {
 		a.rxHaveSeq = true
 		a.rxLastSeq = seq
-		return
+		return true
 	}
 	switch d := int16(seq - a.rxLastSeq); {
 	case d == 0:
+		return false
 	case d < 0:
 		delete(a.rxMissing, seq)
+		return false
 	case d == 1:
 		a.rxLastSeq = seq
+		return true
 	case int(d) <= icnMaxMissing:
 		for f := a.rxLastSeq + 1; f != seq; f++ {
 			a.rxMissing[f] = 0
 		}
 		a.rxLastSeq = seq
+		return true
 	default:
 		a.rxMissing = make(map[uint16]int)
 		a.rxLastSeq = seq
+		return true
 	}
 }