fix(rigctld): stop refusing two commands JTDX sends in Fake It

From a user's log, repeating throughout the session:

	rigctld: split off failed: cat: this radio's backend cannot set split
	rigctld: unimplemented command "X PKTUSB -1"

Both are answered with an error, and JTDX treats a rig-control error as fatal:
it abandoned a transmission 0.86 seconds into a 13.8-second frame.

"S 0" — split OFF. Fake It uses no split, and JTDX still sends this to be sure.
A backend that cannot SET split was refusing a request that was ALREADY
satisfied. It now succeeds when the rig is simplex, without touching the rig at
all. Arming still fails loudly: there the request is real and unmet, and a
client that believes it transmits up the band while the radio sits on the DX's
frequency is exactly what that refusal exists to prevent.

"X <mode>" — set_split_mode. Sent during ordinary setup, Fake It included,
where there is no second VFO to give a mode to. Accepted now: applied when a
split is in force, and a plain success otherwise, because the transmit VFO
already has that mode — it is the same VFO. get_split_mode answers to match.

Both regressions fail without the fix with the exact codes from that log,
RPRT -9 and RPRT -11.

internal/rigctld/split_test.go

@@ -50,3 +50,55 @@ func TestSetSplitRefusalIsReported(t *testing.T) {
 		t.Errorf("a rig that cannot split answered %q — the client will transmit on the wrong frequency", got)
 	}
 }
+
+// "Fake It" uses NO split, and JTDX still sends "S 0" to make sure. A backend
+// that cannot SET split used to answer an error to that — a refusal of a
+// request that was already satisfied — and JTDX read it as rig control failing
+// and abandoned the transmission a second into a 13.8-second frame.
+//
+// Reported against a Yaesu whose backend cannot arm split from software.
+func TestSplitOffOnASimplexRigSucceeds(t *testing.T) {
+	rig := &fakeRig{freq: 14074000, mode: "FT8", noSplit: true}
+	s := New(0, rig, func(string, ...any) {})
+
+	if got, _ := s.handle("S 0 VFOA"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Errorf("split off on a simplex rig answered %q — nothing needed doing", got)
+	}
+	// And it must not have bothered the rig at all.
+	if len(rig.splitCalls) != 0 {
+		t.Errorf("rig was asked to clear a split it did not have: %v", rig.splitCalls)
+	}
+
+	// Arming, however, must still fail loudly: there the request is real and
+	// unmet, and a client that believes it is transmitting up the band while the
+	// radio sits on the DX's frequency is the bug this refusal exists to prevent.
+	if got, _ := s.handle("S 1 VFOB"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Fatalf("arming is deferred to set_split_freq: %q", got)
+	}
+	if got, _ := s.handle("I 14075300.000000"); strings.HasPrefix(got, "RPRT 0") {
+		t.Error("arming split on a backend that cannot must report a failure")
+	}
+}
+
+// X (set_split_mode) is sent by WSJT-X and JTDX during ordinary setup, Fake It
+// included — where there is no second VFO to give a mode to. Answering "not
+// implemented" made JTDX give up on rig control mid-transmission.
+func TestSplitModeIsAccepted(t *testing.T) {
+	rig := &fakeRig{freq: 14074000, mode: "FT8"}
+	s := New(0, rig, func(string, ...any) {})
+
+	// No split: nothing to target, and succeeding is honest — the transmit VFO
+	// already has that mode because it is the same VFO.
+	if got, _ := s.handle("X PKTUSB -1"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Errorf("set_split_mode answered %q", got)
+	}
+	if rig.mode != "FT8" {
+		t.Errorf("the mode was changed with no split in force: %q", rig.mode)
+	}
+
+	// Reading it back must answer a mode and a passband, never an error.
+	got, _ := s.handle("x")
+	if !strings.Contains(got, "FT8") {
+		t.Errorf("get_split_mode = %q, want the current mode", got)
+	}
+}