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/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 {