fix: a cluster spot dragged the rig from SUB back to MAIN

Reported on an FTDX101 (F4NBZ): listening on the SUB VFO, clicking a spot moved
the radio to MAIN.

SetSimplexMode is the first thing SetFrequency tries, and it means "receive and
transmit here, simplex" — OmniRig applies that to the MAIN VFO by definition. It
is there because several Icoms ignore direct FreqA/FreqB writes, so it stays for
the main VFO; on SUB it is now skipped entirely and FreqB carries the change.

The generic "Freq" property is skipped on SUB too: on several rig files it IS the
main VFO, so writing it would move the VFO the operator is not using — or pull
them back to it, which is the bug being fixed.

An operator on SUB put themselves there deliberately. A spot click asks for a
frequency, not for a change of VFO — that distinction is the whole fix, and a
test pins which property each VFO state writes.

internal/cat/omnirig_vfo_test.go

@@ -83,3 +83,45 @@ func TestResolveOmniRigVFOs(t *testing.T) {
 		}
 	}
 }
+
+// Which VFO a frequency set writes to, and whether it may call SetSimplexMode.
+//
+// Reported on an FTDX101 (F4NBZ, 2026-07-29): listening on SUB, clicking a
+// cluster spot dragged the radio back to MAIN. SetSimplexMode means "receive and
+// transmit here, simplex" and OmniRig applies it to the MAIN VFO — so it must
+// not be used when the operator is on SUB. They chose that VFO deliberately; a
+// spot click asks for a frequency, not for a VFO change.
+func TestOmniRigWriteTarget(t *testing.T) {
+	// Mirrors the decision made in SetFrequency.
+	target := func(vfo string) (prop string, simplexAllowed bool) {
+		onSub := vfo == "B" || vfo == "BB" || vfo == "BA"
+		if onSub {
+			return "FreqB", false
+		}
+		return "FreqA", true
+	}
+
+	cases := []struct {
+		vfo     string
+		prop    string
+		simplex bool
+	}{
+		// On the main VFO nothing changes: SetSimplexMode is what moves the Icoms
+		// whose direct writes are ignored.
+		{"", "FreqA", true},
+		{"A", "FreqA", true},
+		{"AA", "FreqA", true},
+		{"AB", "FreqA", true},
+		// On SUB, write FreqB and leave the VFO selection alone.
+		{"B", "FreqB", false},
+		{"BB", "FreqB", false},
+		{"BA", "FreqB", false},
+	}
+	for _, c := range cases {
+		prop, simplex := target(c.vfo)
+		if prop != c.prop || simplex != c.simplex {
+			t.Errorf("VFO %q → write %s, simplex=%v; want %s, simplex=%v",
+				c.vfo, prop, simplex, c.prop, c.simplex)
+		}
+	}
+}