feat(cat): per-rig Kenwood/Elecraft data-mode choice (USB / MD6 / keep)

No single Kenwood mode digit fits every rig for a data mode: a K3/K4 wants
DATA (MD6), a TS-590SG/TS-990S data mode is a USB modifier set on the rig,
and MD6 on a plain Kenwood is FSK/RTTY. So the operator now chooses in
Settings → CAT: USB (default), DATA (MD6), or leave the rig's mode unchanged.

Only soundcard/data modes (FT8, PSK, JT…) follow this; RTTY/FSK stays its own
native mode, and voice/CW are untouched (isKenwoodDataMode + test).

internal/cat/kenwood.go

@@ -56,6 +56,12 @@ type Kenwood struct {
 	// a wire, which is how most operators actually put a rig on the network.
 	host    string
 	digital string // mode name logged for data (FT8 by default)
+	// dataMode chooses what CAT mode a DATA/digital mode (FT8, PSK…) sets, because
+	// no single Kenwood mode digit is right for every rig: "usb" → USB (the SSB-data
+	// base, default), "data" → the DATA mode digit MD6 (Elecraft K3/K4), "keep" →
+	// leave the rig's current mode untouched (safest for a TS-590SG/TS-990S whose
+	// data mode is a USB modifier the operator sets on the rig). See SetMode.
+	dataMode string
 
 	mu   sync.Mutex
 	port serial.Port
@@ -360,6 +366,17 @@ func (k *Kenwood) SetMode(mode string) error {
 	if k.port == nil {
 		return fmt.Errorf("kenwood: not connected")
 	}
+	// Honour the operator's DATA-mode preference for a digital mode. No single
+	// mode digit fits every rig, so the operator picks: keep the rig's mode, use
+	// MD6 (K3/K4 DATA), or fall through to the default USB from kenwoodModeDigit.
+	if isKenwoodDataMode(mode) {
+		switch k.dataMode {
+		case "keep":
+			return nil // leave whatever data mode the operator set on the rig
+		case "data":
+			return k.write("MD6;") // Elecraft K3/K4 DATA mode
+		}
+	}
 	d := kenwoodModeDigit(mode, k.curFreq)
 	if d == 0 {
 		return fmt.Errorf("kenwood: no CAT mode for %q", mode)
@@ -367,6 +384,25 @@ func (k *Kenwood) SetMode(mode string) error {
 	return k.write(fmt.Sprintf("MD%c;", d))
 }
 
+// SetDataMode configures how a DATA/digital mode is set: "usb" (default), "data"
+// (MD6, Elecraft K3/K4) or "keep" (don't change the rig's mode).
+func (k *Kenwood) SetDataMode(m string) {
+	k.mu.Lock()
+	k.dataMode = strings.ToLower(strings.TrimSpace(m))
+	k.mu.Unlock()
+}
+
+// isKenwoodDataMode reports whether a mode name is a soundcard/data mode (FT8,
+// PSK, JT…) rather than a voice/CW/RTTY mode the rig sets natively.
+func isKenwoodDataMode(mode string) bool {
+	switch strings.ToUpper(strings.TrimSpace(mode)) {
+	case "", "LSB", "USB", "SSB", "CW", "CW-R", "CWR", "FM", "NFM", "AM",
+		"RTTY", "FSK", "RTTY-R", "FSK-R":
+		return false
+	}
+	return true
+}
+
 func (k *Kenwood) SetPTT(on bool) error {
 	k.mu.Lock()
 	defer k.mu.Unlock()

internal/cat/kenwood_test.go

@@ -111,6 +111,23 @@ func TestKenwoodModeDigit(t *testing.T) {
 	}
 }
 
+// The DATA-mode preference only applies to soundcard/data modes, never to voice,
+// CW or native RTTY — getting this wrong would hijack an SSB or CW mode change.
+func TestIsKenwoodDataMode(t *testing.T) {
+	data := []string{"FT8", "FT4", "PSK31", "JT65", "DATA", "DIGITAL", "OLIVIA"}
+	notData := []string{"", "LSB", "USB", "SSB", "CW", "CW-R", "FM", "AM", "RTTY", "FSK", "RTTY-R"}
+	for _, m := range data {
+		if !isKenwoodDataMode(m) {
+			t.Errorf("isKenwoodDataMode(%q) = false, want true", m)
+		}
+	}
+	for _, m := range notData {
+		if isKenwoodDataMode(m) {
+			t.Errorf("isKenwoodDataMode(%q) = true, want false", m)
+		}
+	}
+}
+
 // What the log records for each mode digit the rig reports.
 func TestKenwoodModeToADIF(t *testing.T) {
 	cases := []struct {