feat: native Kenwood CAT backend

A sixth CAT backend, talking to a TS-590/890/990/2000 over its serial port with
no OmniRig in between — frequency, mode, VFO, split and PTT. Elecraft K3/K4 and
the "Kenwood" setting on other radios speak the same dialect.

The protocol was not researched from scratch: internal/catemu already ANSWERS
these commands, pretending to be a TS-2000 so an ACOM amplifier follows OpsLog.
The IF frame layout here is read from the same format string catemu emits, so
the two halves of the repository agree by construction — and the test caught my
own fixture being one character short, which is exactly the error that layout
invites.

Design notes worth keeping:

  - IF; is the poll. One frame carries frequency, TX state, mode, VFO and split,
    so simplex operation costs a single round trip; the other VFO is only asked
    for when split is actually on.
  - FA/FB take ELEVEN digits here where Yaesu uses nine. That is the likeliest
    place to copy the Yaesu backend and be wrong by a factor of a hundred, so it
    has its own test.
  - Every lesson the Yaesu backend learned the hard way is built in from the
    start: replies matched to the command that asked, "?;" remembered so a poll
    stops paying a timeout for an unsupported command, the serial handle closed
    before reopening, and a rig that answers nothing reported as absent rather
    than "connected".

Untested on hardware — I have no Kenwood here. The frame parsing is covered by
tests against catemu's own format.

app.go

@@ -116,6 +116,8 @@ const (
 	keyCATXieguAddr       = "cat.xiegu.addr"      // Xiegu CI-V address (factory 0x70)
 	keyCATYaesuPort       = "cat.yaesu.port"      // Yaesu CAT serial port (e.g. COM4)
 	keyCATYaesuBaud       = "cat.yaesu.baud"      // Yaesu CAT baud (FTDX10/101 default 38400)
+	keyCATKenwoodPort     = "cat.kenwood.port"    // Kenwood CAT serial port (TS-590/890/2000, Elecraft)
+	keyCATKenwoodBaud     = "cat.kenwood.baud"    // Kenwood CAT baud (TS-590 default 9600, TS-890 115200)
 	keyCATIcomPort        = "cat.icom.port"       // Icom USB CI-V serial port (e.g. COM5)
 	keyCATIcomBaud        = "cat.icom.baud"       // Icom CI-V baud (default 115200)
 	keyCATIcomAddr        = "cat.icom.addr"       // Icom CI-V address, decimal (IC-7610 = 152 / 0x98)
@@ -357,6 +359,8 @@ type CATSettings struct {
 	XieguAddr       int    `json:"xiegu_addr"`        // Xiegu CI-V address (factory 0x70)
 	YaesuPort       string `json:"yaesu_port"`        // Yaesu CAT serial port (e.g. COM4)
 	YaesuBaud       int    `json:"yaesu_baud"`        // Yaesu CAT baud (FTDX10/101 default 38400)
+	KenwoodPort     string `json:"kenwood_port"`      // Kenwood CAT serial port (TS-590/890/2000, Elecraft)
+	KenwoodBaud     int    `json:"kenwood_baud"`      // Kenwood CAT baud (TS-590 default 9600)
 	IcomPort        string `json:"icom_port"`         // Icom USB CI-V serial port (e.g. COM5)
 	IcomBaud        int    `json:"icom_baud"`         // Icom CI-V baud (default 115200)
 	IcomAddr        int    `json:"icom_addr"`         // Icom CI-V address, decimal (IC-7610 = 152)
@@ -6610,7 +6614,7 @@ func (a *App) GetCATSettings() (CATSettings, error) {
 	if a.settings == nil {
 		return CATSettings{Backend: "omnirig", OmniRigNum: 1, PollMs: 250}, fmt.Errorf("db not initialized")
 	}
-	m, err := a.settings.GetMany(a.ctx, keyCATEnabled, keyCATBackend, keyCATOmniRigNum, keyCATOmniRigVFO, keyCATFlexHost, keyCATFlexPort, keyCATFlexSpots, keyCATFlexDecodeSpots, keyCATFlexDecodeSecs, keyCATXieguPort, keyCATXieguBaud, keyCATXieguAddr, keyCATYaesuPort, keyCATYaesuBaud, keyCATIcomPort, keyCATIcomBaud, keyCATIcomAddr, keyCATIcomNetHost, keyCATIcomNetUser, keyCATIcomNetPass, keyCATIcomNetAudio, keyCATTCIHost, keyCATTCIPort, keyCATTCISpots, keyCATPollMs, keyCATDelayMs, keyCATDigitalDefault, keyCATShareEnabled, keyCATSharePort)
+	m, err := a.settings.GetMany(a.ctx, keyCATEnabled, keyCATBackend, keyCATOmniRigNum, keyCATOmniRigVFO, keyCATFlexHost, keyCATFlexPort, keyCATFlexSpots, keyCATFlexDecodeSpots, keyCATFlexDecodeSecs, keyCATXieguPort, keyCATXieguBaud, keyCATXieguAddr, keyCATYaesuPort, keyCATYaesuBaud, keyCATKenwoodPort, keyCATKenwoodBaud, keyCATIcomPort, keyCATIcomBaud, keyCATIcomAddr, keyCATIcomNetHost, keyCATIcomNetUser, keyCATIcomNetPass, keyCATIcomNetAudio, keyCATTCIHost, keyCATTCIPort, keyCATTCISpots, keyCATPollMs, keyCATDelayMs, keyCATDigitalDefault, keyCATShareEnabled, keyCATSharePort)
 	if err != nil {
 		return CATSettings{}, err
 	}
@@ -6628,6 +6632,8 @@ func (a *App) GetCATSettings() (CATSettings, error) {
 		XieguAddr:       cat.XieguDefaultAddr,
 		YaesuPort:       m[keyCATYaesuPort],
 		YaesuBaud:       38400,
+		KenwoodPort:     m[keyCATKenwoodPort],
+		KenwoodBaud:     9600,
 		IcomPort:        m[keyCATIcomPort],
 		IcomBaud:        115200,
 		IcomAddr:        0x98, // IC-7610 default
@@ -6665,6 +6671,9 @@ func (a *App) GetCATSettings() (CATSettings, error) {
 	if n, _ := strconv.Atoi(m[keyCATYaesuBaud]); n > 0 {
 		out.YaesuBaud = n
 	}
+	if n, _ := strconv.Atoi(m[keyCATKenwoodBaud]); n > 0 {
+		out.KenwoodBaud = n
+	}
 	if n, _ := strconv.Atoi(m[keyCATIcomBaud]); n > 0 {
 		out.IcomBaud = n
 	}
@@ -6718,6 +6727,9 @@ func (a *App) SaveCATSettings(s CATSettings) error {
 	if s.YaesuBaud <= 0 {
 		s.YaesuBaud = 38400
 	}
+	if s.KenwoodBaud <= 0 {
+		s.KenwoodBaud = 9600
+	}
 	if s.IcomBaud <= 0 {
 		s.IcomBaud = 115200
 	}
@@ -6775,6 +6787,8 @@ func (a *App) SaveCATSettings(s CATSettings) error {
 		keyCATXieguAddr:       strconv.Itoa(s.XieguAddr),
 		keyCATYaesuPort:       strings.TrimSpace(s.YaesuPort),
 		keyCATYaesuBaud:       strconv.Itoa(s.YaesuBaud),
+		keyCATKenwoodPort:     strings.TrimSpace(s.KenwoodPort),
+		keyCATKenwoodBaud:     strconv.Itoa(s.KenwoodBaud),
 		keyCATIcomPort:        strings.TrimSpace(s.IcomPort),
 		keyCATIcomBaud:        strconv.Itoa(s.IcomBaud),
 		keyCATIcomAddr:        strconv.Itoa(s.IcomAddr),
@@ -12201,6 +12215,12 @@ func (a *App) reloadCAT() {
 		// (a rig file that hides the VFO, a Freq property meaning A on one model
 		// and B on another); talking to the radio directly removes it.
 		a.cat.Start(cat.NewYaesu(s.YaesuPort, s.YaesuBaud, s.DigitalDefault))
+	case "kenwood":
+		// Native Kenwood CAT — TS-590/890/990/2000 and everything that speaks the
+		// same dialect (Elecraft K3/K4, and the "Kenwood" setting on other rigs).
+		// One IF; frame carries frequency, mode, VFO and split, so the poll costs a
+		// single round trip where OmniRig needed a rig file to describe each one.
+		a.cat.Start(cat.NewKenwood(s.KenwoodPort, s.KenwoodBaud, s.DigitalDefault))
 	case "icom":
 		// Native Icom CI-V over the radio's USB serial port (local control).
 		// Same civ protocol the network backend reuses for remote.