fix: Yaesu mode commands always addressed the main receiver

Reported, and reasoned out from the half that already worked: a spot click now
tunes the right VFO, but it set the mode with MD0 — main — whatever VFO the
operator was on. Working from SUB, the spot changed the mode of the VFO NOT in
use and left the one in use as it was. The author inferred the sub half from
seeing main change, and was right.

The read had the same fault, so the console displayed main's mode while showing
the sub VFO's frequency.

Mode now addresses the receiver in use — MD0 for main, MD1 for sub — in the poll
read, in SetMode, and in the console's raw-mode setter. A rig without a sub
receiver never sees MD1: curVFO only becomes B where the rig reports its receive
VFO in the first place.

internal/cat/yaesu.go

@@ -17,7 +17,7 @@ package cat
 //
 //	FA;            → FA014074000;   VFO A frequency, 9 digits, Hz
 //	FB;            → FB014100000;   VFO B frequency
-//	MD0;           → MD02;          operating mode of the main receiver
+//	MD0;/MD1;      → MD02;          operating mode — 0 = main receiver, 1 = sub
 //	FR;            → FR1;           RECEIVE VFO (0=main/A, 1=sub/B)
 //	VS;            → VS0;           selected VFO, on models without FR
 //	ST;            → ST1;           split (FTDX10/FTDX101)
@@ -304,7 +304,7 @@ func (y *Yaesu) ReadState() (RigState, error) {
 		y.curRXFreq = s.RxFreqHz
 	}
 
-	if r, err := y.ask("MD0;"); err == nil && len(r) >= 4 {
+	if r, err := y.ask("MD" + y.modeVFOSuffix() + ";"); err == nil && len(r) >= 4 {
 		// Keep the RAW mode too: ADIF folds CW-U/CW-L and DATA-U/DATA-L together,
 		// but the panel has to show which sideband the rig is actually on.
 		y.panel.RawMode = yaesuRawModeName(r[3])
@@ -351,7 +351,25 @@ func (y *Yaesu) SetMode(mode string) error {
 	if d == 0 {
 		return fmt.Errorf("yaesu: no CAT mode for %q", mode)
 	}
-	return y.write(fmt.Sprintf("MD0%c;", d))
+	return y.write(fmt.Sprintf("MD%s%c;", y.modeVFOSuffix(), d))
+}
+
+// modeVFOSuffix picks which receiver the mode commands address: "0" is main,
+// "1" is sub.
+//
+// Both the read and the write used 0 unconditionally. A spot click already tuned
+// the right VFO — that part was fixed — but then set the MODE on main, so an
+// operator working from the sub receiver had a spot change the mode of the VFO
+// they were not using and leave the one they were on untouched (F4NBZ,
+// 2026-07-29). The rig reads the same way, so the panel showed main's mode too.
+//
+// Rigs without a sub receiver simply never see "1": curVFO only becomes B on a
+// rig that reports its receive VFO.
+func (y *Yaesu) modeVFOSuffix() string {
+	if y.curVFO == "B" {
+		return "1"
+	}
+	return "0"
 }
 
 func (y *Yaesu) SetPTT(on bool) error {