fix(catshare): stop FT8 dial drift in shared CAT (rigctld freq echo)

A digital app (JTDX/WSJT-X) sharing OpsLog's rig in "Fake It" split follows
the dial by polling get_freq. Freq() is the last polled value and lags a
set_freq by a poll cycle, so right after a transmission the client read the
still-shifted frequency, mistook it for a manual QSY and adopted it — the dial
crept down every over and never came back. get_freq now echoes the last
commanded frequency until the rig confirms it (or a short deadline passes),
closing the race. Backend-agnostic, so it fixes every rig, not just Kenwood.

internal/rigctld/rigctld.go

@@ -59,8 +59,28 @@ type Server struct {
 	ln     net.Listener
 	conns  map[net.Conn]struct{}
 	closed bool
+
+	// Optimistic frequency echo. A sharing client that follows our dial by
+	// polling get_freq (WSJT-X / JTDX in "Fake It" split) shifts the frequency on
+	// TX and restores it on RX. Freq() is the last value POLLED from the rig, and
+	// it lags a set_freq by up to a poll cycle (~100-200 ms). In that window the
+	// client — no longer transmitting — reads back the still-shifted frequency,
+	// mistakes it for a manual QSY and adopts it, so every over creeps the dial by
+	// the shift amount and it never comes back. Echoing the last commanded
+	// frequency until the rig confirms it (or a short deadline passes) closes the
+	// window: the client always reads exactly what it just set.
+	echoMu   sync.Mutex
+	echoHz   int64
+	echoAt   time.Time
+	echoWant bool
 }
 
+// freqEchoTTL caps how long a commanded frequency is echoed when the rig never
+// reports it back (e.g. it rounded to a coarser step). Long enough to cover a
+// poll cycle with margin, short enough that a genuine knob turn during the
+// window surfaces quickly.
+const freqEchoTTL = 2 * time.Second
+
 func New(port int, rig Rig, logf func(string, ...any)) *Server {
 	if port <= 0 || port > 65535 {
 		port = 4532 // the rigctld default every client pre-fills
@@ -193,7 +213,7 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 		return "", true
 
 	case "f", "\\get_freq":
-		return fmt.Sprintf("%d\n", s.rig.Freq()), false
+		return fmt.Sprintf("%d\n", s.reportedFreq()), false
 	case "F", "\\set_freq":
 		if len(args) < 1 {
 			return rprt(-1), false
@@ -210,6 +230,7 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 			s.log("rigctld: set_freq %d failed: %v", hz, err)
 			return rprt(-9), false
 		}
+		s.noteSetFreq(hz)
 		return rprt(0), false
 
 	case "m", "\\get_mode":
@@ -263,7 +284,7 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 	case "i", "\\get_split_freq":
 		_, tx := s.rig.Split()
 		if tx <= 0 {
-			tx = s.rig.Freq()
+			tx = s.reportedFreq()
 		}
 		return fmt.Sprintf("%d\n", tx), false
 	case "I", "\\set_split_freq":
@@ -279,6 +300,30 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 
 func rprt(code int) string { return fmt.Sprintf("RPRT %d\n", code) }
 
+// noteSetFreq records a frequency a client just commanded, so the next reads
+// echo it back until the rig confirms the tune.
+func (s *Server) noteSetFreq(hz int64) {
+	s.echoMu.Lock()
+	s.echoHz, s.echoAt, s.echoWant = hz, time.Now(), true
+	s.echoMu.Unlock()
+}
+
+// reportedFreq is what get_freq answers: the last commanded frequency while the
+// rig is still catching up to it, otherwise the live polled value. See echoWant.
+func (s *Server) reportedFreq() int64 {
+	live := s.rig.Freq()
+	s.echoMu.Lock()
+	defer s.echoMu.Unlock()
+	if s.echoWant {
+		if live == s.echoHz || time.Since(s.echoAt) > freqEchoTTL {
+			s.echoWant = false // rig confirmed the tune, or we waited long enough
+			return live
+		}
+		return s.echoHz
+	}
+	return live
+}
+
 // stripVFOArg drops a leading VFO name from a command's arguments.
 //
 // Hamlib has two dialects. In the plain one a client sends "F 14074000"; in VFO