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