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.

internal/cat/kenwood_loopback_test.go

@@ -108,7 +108,13 @@ type ts2000 struct {
 	mode       byte
 	onB        bool
 	split      bool
-	seen       []string
+	// lazyIF models a rig that answers FR/FT correctly but never fills IF's
+	// split bit — the behaviour reported on a Flex through its Kenwood CAT
+	// emulation, where the frequency reads perfectly and split never appears.
+	lazyIF bool
+	// noVFOCmds models a rig that rejects FR/FT outright ("?;").
+	noVFOCmds bool
+	seen      []string
 }
 
 func (r *ts2000) answer(cmd string) string {
@@ -127,7 +133,7 @@ func (r *ts2000) answer(cmd string) string {
 		return fmt.Sprintf("FB%011d;", r.vfoB)
 	case cmd == "IF;":
 		split := byte('0')
-		if r.split {
+		if r.split && !r.lazyIF {
 			split = '1'
 		}
 		// The catemu layout: IF | freq(11) | step(4) | RIT(±5) | 3 | mem(2) |
@@ -143,6 +149,21 @@ func (r *ts2000) answer(cmd string) string {
 	case strings.HasPrefix(cmd, "MD") && len(cmd) == 4:
 		r.mode = cmd[2]
 		return ""
+	case cmd == "FR;" || cmd == "FT;":
+		if r.noVFOCmds {
+			return "?;" // rejected, as a rig that does not know the command answers
+		}
+		// FR is the receive VFO, FT the transmit one. They differ exactly when
+		// the rig is in split.
+		v := vfoDigit
+		if cmd == "FT;" && r.split {
+			if vfoDigit == '0' {
+				v = '1'
+			} else {
+				v = '0'
+			}
+		}
+		return fmt.Sprintf("%s%c;", strings.TrimSuffix(cmd, ";"), v)
 	}
 	return "" // AI0;, TX;, RX; — set commands, no reply, as on a real rig
 }
@@ -251,3 +272,86 @@ func TestKenwoodSilentRigIsNotConnected(t *testing.T) {
 		t.Errorf("a stale model survived a failed connect: %q", k.model)
 	}
 }
+
+// Split found through FR/FT when the rig never fills IF's split bit.
+//
+// Reported on a Flex through its Kenwood CAT emulation: frequency read
+// perfectly, split never appeared. Rather than guess at which IF column that
+// firmware populates, ask the question that DEFINES split — is the transmit VFO
+// a different VFO from the receive one — which is what FR and FT answer, and
+// the same rule the Yaesu backend settled on after several wrong turns.
+func TestKenwoodSplitFromFRFT(t *testing.T) {
+	rig := &ts2000{vfoA: 14250000, vfoB: 14260000, mode: '2', lazyIF: true}
+	k := NewKenwood("COM-TEST", 9600, "FT8")
+	k.dialPort = dialTo(rig)
+	if err := k.Connect(); err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer k.Disconnect()
+
+	// Simplex: FR and FT agree, and nothing may be invented from that.
+	s, err := k.ReadState()
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if s.Split {
+		t.Errorf("split reported while FR and FT agree: tx=%d rx=%d", s.FreqHz, s.RxFreqHz)
+	}
+
+	// Split on, IF still silent about it: receive on A, transmit on B.
+	rig.split = true
+	if s, err = k.ReadState(); err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if !s.Split {
+		t.Fatal("split not detected — FR/FT disagreed and IF's bit was empty, which is the reported case")
+	}
+	if s.FreqHz != 14260000 || s.RxFreqHz != 14250000 {
+		t.Errorf("tx=%d rx=%d — want tx 14260000 (B), rx 14250000 (A)", s.FreqHz, s.RxFreqHz)
+	}
+}
+
+// A rig that rejects FR/FT is asked once, then left alone — and its IF split
+// bit still works. The fallback must not cost a timeout on every poll, nor
+// break the rigs that were already fine.
+func TestKenwoodSplitWhenFRFTRejected(t *testing.T) {
+	rig := &ts2000{vfoA: 14250000, vfoB: 14260000, mode: '2', noVFOCmds: true}
+	k := NewKenwood("COM-TEST", 9600, "FT8")
+	k.dialPort = dialTo(rig)
+	if err := k.Connect(); err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer k.Disconnect()
+
+	if _, err := k.ReadState(); err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	asked := 0
+	for _, c := range rig.seen {
+		if c == "FR;" || c == "FT;" {
+			asked++
+		}
+	}
+	if _, err := k.ReadState(); err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	after := 0
+	for _, c := range rig.seen {
+		if c == "FR;" || c == "FT;" {
+			after++
+		}
+	}
+	if after != asked {
+		t.Errorf("a rejected command was asked again: %d then %d", asked, after)
+	}
+
+	// IF's own split bit still drives the result on such a rig.
+	rig.split = true
+	s, err := k.ReadState()
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if !s.Split || s.FreqHz != 14260000 || s.RxFreqHz != 14250000 {
+		t.Errorf("IF-reported split broke: split=%v tx=%d rx=%d", s.Split, s.FreqHz, s.RxFreqHz)
+	}
+}