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/cat/cat.go

@@ -200,6 +200,31 @@ func (m *Manager) SetPTT(on bool) error {
 	return m.exec(func(b Backend) error { return b.SetPTT(on) })
 }
 
+// splitSetter is implemented by the backends that can arm split AND place the
+// transmit frequency. Both together: arming without setting the dial transmits
+// on whatever the transmit VFO happened to hold, which is worse than refusing.
+type splitSetter interface {
+	SetSplit(on bool, txHz int64) error
+}
+
+// SetSplit arms or clears split on the rig, with the transmit frequency.
+//
+// Returns a plain error on a backend that cannot do it, and that is the point.
+// The shared-CAT server used to answer "done" to WSJT-X's split commands while
+// doing nothing at all — the software then believed it was transmitting up the
+// band when it was transmitting on the DX's own frequency. A refusal WSJT-X can
+// report is worth far more than a success it cannot check.
+func (m *Manager) SetSplit(on bool, txHz int64) error {
+	return m.exec(func(b Backend) error {
+		s, ok := b.(splitSetter)
+		if !ok {
+			return fmt.Errorf("cat: this radio's backend cannot set split from software — " +
+				"use Split Operating: Fake It in WSJT-X/JTDX, or set split on the radio itself")
+		}
+		return s.SetSplit(on, txHz)
+	})
+}
+
 // SpotInfo is one cluster spot to render on a backend that supports a spot
 // overlay (the FlexRadio panadapter). Color is an optional "#AARRGGBB" string;
 // the backend picks a default when it's empty. (Status-based colouring can be

internal/cat/flex.go

@@ -59,15 +59,15 @@ type Flex struct {
 	meterRawLogged bool         // log the first raw meter-definition status once
 	txRawLogged    bool         // log the first raw transmit status once (field-name audit)
 
-	spotsEnabled bool           // push cluster spots + manage the panadapter overlay
-	spotIdx      map[int]bool   // panadapter spot indices currently known to the radio
+	spotsEnabled    bool           // push cluster spots + manage the panadapter overlay
+	spotIdx         map[int]bool   // panadapter spot indices currently known to the radio
 	pendingSpot     map[int]string // seq → callsign, awaiting the spot index in the R response
 	pendingSpotMode map[int]string // seq → ADIF mode, paired with pendingSpot
 	pendingSplit    map[int]bool   // seq → awaiting the new TX slice's index (split create)
 	spotCall        map[int]string // spot index → callsign (to fill the call on a panadapter click)
 	spotMode        map[int]string // spot index → ADIF mode, so a click can also set the slice mode (SmartSDR tunes the spot's freq but not its mode)
 	spotByCall      map[string]int // callsign → live spot index, so re-spotting a call replaces its old spot (WSJT decodes re-fire every cycle)
-	sentCmds     map[int]string // seq → command text, so an R<seq> error names the command
+	sentCmds        map[int]string // seq → command text, so an R<seq> error names the command
 
 	// OnSpotClick is called (off the reader goroutine's hot path) when the user
 	// clicks one of our spots on the panadapter, with the spot's callsign and

internal/cat/kenwood.go

@@ -513,6 +513,43 @@ func isKenwoodDataMode(mode string) bool {
 	return true
 }
 
+// SetSplit arms or clears split, and when arming puts txHz on the transmit VFO.
+//
+// Both halves in one call on purpose. WSJT-X sends "split on, VFO B" and "VFO B
+// to 14075300" as two commands, and honouring only the first is worse than
+// honouring neither: split would arm on whatever VFO B happened to hold, so the
+// operator transmits somewhere they never chose while the software reports
+// exactly what they asked for. Nothing is armed here until the frequency is on
+// the dial.
+//
+// FR selects the receive VFO, FT the transmit one — the same pair the poll loop
+// already reads to detect split, so this writes what State() knows how to read.
+func (k *Kenwood) SetSplit(on bool, txHz int64) error {
+	k.mu.Lock()
+	defer k.mu.Unlock()
+	if k.port == nil {
+		return fmt.Errorf("kenwood: not connected")
+	}
+	if !on {
+		// Transmit follows receive again. FR is left alone: which VFO the operator
+		// listens on is theirs to choose, and clearing split should not move them.
+		return k.write("FT0;")
+	}
+	if txHz <= 0 || txHz > 99_999_999_999 {
+		return fmt.Errorf("kenwood: split TX frequency %d out of the 11-digit CAT range", txHz)
+	}
+	// The transmit dial FIRST, then arm. Arming first would transmit on the old
+	// contents of VFO B for however long the next command takes to arrive — brief,
+	// but on the wrong frequency, and this runs the instant before a transmission.
+	if err := k.write(fmt.Sprintf("FB%011d;", txHz)); err != nil {
+		return err
+	}
+	if err := k.write("FR0;"); err != nil { // receive on A
+		return err
+	}
+	return k.write("FT1;") // transmit on B
+}
+
 func (k *Kenwood) SetPTT(on bool) error {
 	k.mu.Lock()
 	defer k.mu.Unlock()

internal/cat/kenwood_test.go

@@ -102,7 +102,7 @@ func TestKenwoodModeDigit(t *testing.T) {
 		{"FM", 145000000, '4'},
 		{"FT8", 7074000, '2'},  // data is ALWAYS USB, even below 10 MHz (K3 "DATA REV" otherwise)
 		{"FT8", 14074000, '2'}, // …and above
-		{"", 14074000, 0}, // nothing to set
+		{"", 14074000, 0},      // nothing to set
 	}
 	for _, c := range cases {
 		if got := kenwoodModeDigit(c.mode, c.hz); got != c.want {