feat(rigctld): split that actually reaches the radio, or an honest refusal

set_split_vfo and set_split_freq both answered RPRT 0 and did nothing. WSJT-X
and JTDX in "Split Operating: Rig" send exactly that pair, believed both, and
transmitted on the RECEIVE frequency — on a pileup, straight onto the DX, while
showing the operator precisely what they had asked for. A lie that leaves no
trace in any log is the worst kind of bug this program can have.

The two commands are honoured as a PAIR. Arming alone does nothing on the radio,
because WSJT-X sends the frequency second and split armed on whatever the
transmit VFO happened to hold is worse than no split at all: it transmits
somewhere the operator never chose. The request is remembered and set_split_freq
does the work.

Kenwood gains SetSplit — FB to place the dial, then FR0/FT1 to arm, in that
order for the same reason. It writes what State() already knows how to read.

Everything else REFUSES, and that is the feature, not a shortfall. Only Flex and
Icom could even toggle split before, neither could set the transmit frequency,
and Yaesu, TCI and OmniRig have nothing at all. A refusal WSJT-X can report —
and act on, by falling back to Fake It — is worth far more than a success it has
no way to check.

Both paths are pinned: split reaching the rig as one armed call with the right
frequency, and a backend that cannot do it producing an error rather than RPRT 0.

internal/rigctld/rigctld.go

@@ -49,6 +49,10 @@ type Rig interface {
 	SetFreq(hz int64) error
 	SetMode(mode string) error
 	SetPTT(on bool) error
+	// SetSplit arms or clears split and places the transmit frequency. Returns an
+	// error on a rig that cannot: a refusal the client can report is worth far
+	// more than a success it has no way to check.
+	SetSplit(on bool, txHz int64) error
 }
 
 type Server struct {
@@ -76,6 +80,9 @@ type Server struct {
 	// be told from the very first call — where the radio's state is unknown and
 	// the command must go through.
 	pttKnown atomic.Bool
+	// splitWanted remembers a set_split_vfo that arrived before the frequency it
+	// needs, so the pair can be honoured in the order the client sends them.
+	splitWanted atomic.Bool
 }
 
 func New(port int, rig Rig, logf func(string, ...any)) *Server {
@@ -383,7 +390,31 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 		}
 		return fmt.Sprintf("%d\nVFOB\n", n), false
 	case "S", "\\set_split_vfo":
-		return rprt(0), false // see set_vfo — split is driven from the rig
+		// "S <0|1> <VFO>". The VFO argument is ignored: which dial transmits is the
+		// rig's own business, and every backend here puts it on the second one.
+		//
+		// This used to answer RPRT 0 and do NOTHING. WSJT-X in "Split Operating:
+		// Rig" sends this and set_split_freq, believed both, and transmitted on the
+		// RECEIVE frequency — on a pileup, straight onto the DX, while the software
+		// showed exactly what the operator had asked for. A lie that leaves no
+		// trace anywhere is the worst kind of bug, so it now works or says so.
+		if len(args) < 1 {
+			return rprt(-1), false
+		}
+		if args[0] != "0" {
+			// Arming needs a frequency, and WSJT-X sends set_split_freq AFTER this.
+			// Remember the request and let that command do the work: alone, this
+			// would arm split on whatever the transmit VFO happens to hold.
+			s.splitWanted.Store(true)
+			return rprt(0), false
+		}
+		s.splitWanted.Store(false)
+		if err := s.rig.SetSplit(false, 0); err != nil {
+			s.log("rigctld: split off failed: %v", err)
+			return rprt(-9), false
+		}
+		s.log("rigctld: split off")
+		return rprt(0), false
 	case "i", "\\get_split_freq":
 		_, tx := s.rig.Split()
 		if tx <= 0 {
@@ -391,6 +422,20 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 		}
 		return fmt.Sprintf("%d\n", tx), false
 	case "I", "\\set_split_freq":
+		if len(args) < 1 {
+			return rprt(-1), false
+		}
+		// Hamlib sends a float ("14075300.000000"), so parse as one.
+		hz, err := strconv.ParseFloat(args[0], 64)
+		if err != nil || hz <= 0 {
+			return rprt(-1), false
+		}
+		if err := s.rig.SetSplit(true, int64(hz)); err != nil {
+			s.log("rigctld: split TX %.0f Hz failed: %v", hz, err)
+			return rprt(-9), false
+		}
+		s.splitWanted.Store(true)
+		s.log("rigctld: split ON, TX %.0f Hz", hz)
 		return rprt(0), false
 
 	default: