feat: Yaesu panel — sideband on re-click, split readout, one-touch up 1/up 5

Three corrections from the operator's second pass.

The sideband gesture was wrong: I used double-click, which hides the action.
Clicking a button that is ALREADY active now flips its sideband — CW-U → CW-L →
CW-U. One button, one finger, nothing to discover.

A lit SPLIT chip does not tell an operator anything useful: it says split is on,
not where they transmit. The header now shows the TX frequency and the offset in
kHz whenever split is active.

And the offset that matters is set in one action: up 1 kHz on CW, up 5 kHz on
phone. Doing it by hand means swapping VFOs, retuning and swapping back — exactly
the fumbling a panel exists to remove. Both are offered rather than picked from
the mode, because which one is idiomatic is the operator's call, and the button
turns split on at the same time.

The offset is measured from the RECEIVE frequency and written to the VFO we are
not listening on, so it stays correct when the operator works on VFO B, where the
roles are mirrored.

internal/cat/yaesu_panel.go

@@ -34,9 +34,13 @@ type YaesuTXState struct {
 
 	Transmitting bool `json:"transmitting"`
 	Split        bool `json:"split"`
-	SMeter       int  `json:"s_meter"`     // 0-100 (raw 0-255)
-	PowerMeter   int  `json:"power_meter"` // 0-100, TX only
-	SWRMeter     int  `json:"swr_meter"`   // 0-100, TX only
+	// SplitTXHz is where the rig will TRANSMIT while split — the panel showed a
+	// lit SPLIT button and nothing else, which does not tell an operator whether
+	// they are up 1 or up 5.
+	SplitTXHz  int64 `json:"split_tx_hz"`
+	SMeter     int   `json:"s_meter"`     // 0-100 (raw 0-255)
+	PowerMeter int   `json:"power_meter"` // 0-100, TX only
+	SWRMeter   int   `json:"swr_meter"`   // 0-100, TX only
 
 	RFPower int    `json:"rf_power"` // watts
 	MicGain int    `json:"mic_gain"` // 0-100
@@ -72,6 +76,7 @@ type YaesuController interface {
 	SetYaesuNarrow(bool) error
 	SetYaesuVOX(bool) error
 	SetYaesuSplit(bool) error
+	SetYaesuSplitOffset(int64) error
 	SetYaesuBand(string) error
 	SetYaesuModeRaw(string) error
 	TuneYaesuATU() error
@@ -88,9 +93,13 @@ func (y *Yaesu) YaesuState() YaesuTXState {
 
 // readPanel refreshes the meters, and the settings on the slower beat. Called
 // from ReadState with the mutex HELD, so it shares the same serialised link.
-func (y *Yaesu) readPanel(mode string, split bool) {
+func (y *Yaesu) readPanel(mode string, split bool, txHz int64) {
 	y.panel.Mode = mode
 	y.panel.Split = split
+	y.panel.SplitTXHz = 0
+	if split {
+		y.panel.SplitTXHz = txHz
+	}
 
 	// TX state first: which meters mean anything depends on it, and a power
 	// reading shown while receiving is how a panel lies.
@@ -485,3 +494,47 @@ func yaesuRawModeName(d byte) string {
 	}
 	return ""
 }
+
+// SetYaesuSplitOffset puts the transmit VFO a fixed distance above the receive
+// one and turns split on, in a single action.
+//
+// This is the split an operator actually uses when working a pile-up: listen on
+// the DX, transmit up 5 kHz on phone or up 1 kHz on CW. Doing it by hand means
+// swapping VFOs, retuning and swapping back, which is exactly the fumbling a
+// panel should remove.
+//
+// The offset is applied to the RECEIVE frequency and written to the OTHER VFO —
+// whichever that is. On VFO B the roles are mirrored, so listening on B writes A.
+func (y *Yaesu) SetYaesuSplitOffset(offsetHz int64) error {
+	y.mu.Lock()
+	defer y.mu.Unlock()
+	if y.port == nil {
+		return fmt.Errorf("yaesu: not connected")
+	}
+	rx := y.curRXFreq
+	if rx <= 0 {
+		return fmt.Errorf("yaesu: no receive frequency read yet")
+	}
+	tx := rx + offsetHz
+	if tx <= 0 || tx > 999_999_999 {
+		return fmt.Errorf("yaesu: split frequency %d out of the CAT range", tx)
+	}
+	// Write the VFO we are NOT listening on.
+	cmd := "FB"
+	if y.curVFO == "B" {
+		cmd = "FA"
+	}
+	if err := y.write(fmt.Sprintf("%s%09d;", cmd, tx)); err != nil {
+		return err
+	}
+	if y.splitCmd == "" {
+		return fmt.Errorf("yaesu: this rig answered neither ST; nor FT; — split cannot be set")
+	}
+	time.Sleep(30 * time.Millisecond)
+	if err := y.write(fmt.Sprintf("%s1;", y.splitCmd)); err != nil {
+		return err
+	}
+	y.panel.Split = true
+	y.panel.SplitTXHz = tx
+	return nil
+}