fix: Kenwood split read from FR/FT when the status frame omits it

Reported from a Flex in Kenwood CAT mode: frequency read perfectly, split never
appeared. Not every rig speaking this dialect fills IF's split bit.

Rather than guess which column that firmware populates, ask the question that
DEFINES split — is the transmit VFO a different VFO from the receive one — which
is exactly what FR and FT answer, and the same rule the Yaesu backend settled on
after several wrong turns. FR is also trusted over IF for which VFO is in use:
they are asked in the same breath, and a rig vague about the split bit may be
just as vague about the VFO field.

Cost is bounded: a rig that rejects FR/FT answers "?;" once and is never asked
again, so this is two short commands per poll only where it works. Both paths
are tested against the emulator — split found through FR/FT with IF silent, and
IF-reported split still working on a rig that refuses FR/FT without re-asking.

Also extends the CAT wire trace to the Kenwood backend, ASCII quoted so an empty
reply is visible as such: "" and ";" look identical unquoted, and telling them
apart is the whole question when a rig half-supports a command. If this fix is
not the whole story on real hardware, the trace is what will say so.

frontend/src/components/SettingsModal.tsx

@@ -2516,7 +2516,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
               </div>
             </>
             )}
-            {(catCfg.backend === 'icom' || catCfg.backend === 'xiegu') && (
+            {['icom', 'xiegu', 'kenwood'].includes(catCfg.backend) && (
               <div className="border-t border-border/60 pt-3">
                 <label className="flex items-center gap-2 text-sm cursor-pointer">
                   <Checkbox checked={civTrace} onCheckedChange={(c) => { setCivTrace(!!c); SetCIVTrace(!!c); }} />