feat(cat): make the DTR/RTS lowering a setting

Both defaults break someone. 0.22.7 lowered the lines on Yaesu and
Kenwood and silenced a TS-990 whose interface needs RTS raised to
transmit; 0.22.8 stopped lowering them and an FT8 station reported its
output power wandering, cured by closing OpsLog — Windows raises both on
open and the interface reads one as PTT.

So it is now a checkbox on those two backends, off by default: that is
how they behaved before any of this. Xiegu keeps lowering them
unconditionally (its own report, its own explicit keying setting), and
the CI-V backend keeps the behaviour it has always had.

internal/cat/yaesu.go

@@ -82,9 +82,12 @@ var yaesuModeToADIF = map[byte]string{
 }
 
 type Yaesu struct {
-	portName string
-	baud     int
-	digital  string // ADIF mode reported for DATA (FT8, RTTY…)
+	// lowerLines deasserts DTR and RTS on connect — see Kenwood.lowerLines for
+	// why neither default is safe for every station.
+	lowerLines bool
+	portName   string
+	baud       int
+	digital    string // ADIF mode reported for DATA (FT8, RTTY…)
 
 	mu   sync.Mutex
 	port serial.Port
@@ -127,6 +130,14 @@ func NewYaesu(portName string, baud int, digital string) *Yaesu {
 	return &Yaesu{portName: strings.TrimSpace(portName), baud: baud, digital: digital, curVFO: "A"}
 }
 
+// SetLowerLines chooses whether DTR and RTS are deasserted on connect. Set
+// before Connect.
+func (y *Yaesu) SetLowerLines(v bool) {
+	y.mu.Lock()
+	y.lowerLines = v
+	y.mu.Unlock()
+}
+
 func (y *Yaesu) Name() string { return "yaesu" }
 
 func (y *Yaesu) Connect() error {
@@ -150,12 +161,12 @@ func (y *Yaesu) Connect() error {
 	if err != nil {
 		return fmt.Errorf("yaesu: open %s @ %d baud: %w", y.portName, y.baud, err)
 	}
-	// The modem lines are LEFT ALONE — see the same note in kenwood.go.
-	//
-	// Deasserting them to keep an interface from keying the rig silenced radios
-	// instead: many USB-serial interfaces will not transmit with RTS low. The
-	// Xiegu fault that change was written for is handled in that backend, which
-	// now has an explicit setting for which line keys the rig.
+	// The modem lines are only touched when the operator asks for it — see the
+	// note on Kenwood.lowerLines. Both defaults break somebody's station.
+	if y.lowerLines {
+		_ = p.SetDTR(false)
+		_ = p.SetRTS(false)
+	}
 	p.SetReadTimeout(300 * time.Millisecond)
 	y.port = p