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/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