feat: serial CW keyer — add invert (active-LOW) polarity option

Some CW interfaces key on the opposite line level (transmit at rest / key
backwards). New "Invert keying (active-LOW)" toggle flips the asserted level for
both the CW and PTT lines, and the idle state drives both lines to their
inactive level accordingly so an active-low rig isn't left keyed at rest.
Default off = N1MM convention (line HIGH = active).

internal/winkeyer/serialcw.go

@@ -39,6 +39,7 @@ var morseTable = map[rune]string{
 type serialKeyer struct {
 	port   serial.Port
 	keyDTR bool // true: CW on DTR, PTT on RTS (N1MM default). false: swapped.
+	invert bool // true: active-LOW — asserting a line means driving it LOW
 	usePTT bool
 	leadMs int
 	tailMs int
@@ -58,6 +59,7 @@ func newSerialKeyer(port serial.Port, cfg Config, onBusy func(bool)) *serialKeye
 	k := &serialKeyer{
 		port:   port,
 		keyDTR: !strings.EqualFold(strings.TrimSpace(cfg.CWKeyLine), "rts"), // default DTR=CW
+		invert: cfg.CWInvert,
 		usePTT: cfg.UsePTT,
 		leadMs: cfg.LeadInMs,
 		tailMs: cfg.TailMs,
@@ -69,17 +71,21 @@ func newSerialKeyer(port serial.Port, cfg Config, onBusy func(bool)) *serialKeye
 		stop:   make(chan struct{}),
 		done:   make(chan struct{}),
 	}
-	k.allLow() // start idle: key up, PTT off
+	k.idle() // start inactive: key up, PTT off
 	go k.run()
 	return k
 }
 
+// level maps a logical "active" state to the physical line level, honouring the
+// invert (active-LOW) option: normally active = HIGH, inverted active = LOW.
+func (k *serialKeyer) level(active bool) bool { return active != k.invert }
+
 // setKey raises/lowers the CW key line; setPTT the PTT line (only when enabled).
 func (k *serialKeyer) setKey(down bool) {
 	if k.keyDTR {
-		_ = k.port.SetDTR(down)
+		_ = k.port.SetDTR(k.level(down))
 	} else {
-		_ = k.port.SetRTS(down)
+		_ = k.port.SetRTS(k.level(down))
 	}
 }
 func (k *serialKeyer) setPTT(on bool) {
@@ -87,12 +93,18 @@ func (k *serialKeyer) setPTT(on bool) {
 		return
 	}
 	if k.keyDTR {
-		_ = k.port.SetRTS(on)
+		_ = k.port.SetRTS(k.level(on))
 	} else {
-		_ = k.port.SetDTR(on)
+		_ = k.port.SetDTR(k.level(on))
 	}
 }
-func (k *serialKeyer) allLow() { _ = k.port.SetDTR(false); _ = k.port.SetRTS(false) }
+
+// idle drives BOTH lines to their inactive level (key up, PTT off), respecting
+// the invert option — so an active-LOW interface isn't left keyed at rest.
+func (k *serialKeyer) idle() {
+	_ = k.port.SetDTR(k.level(false))
+	_ = k.port.SetRTS(k.level(false))
+}
 
 // SetSpeed / Send / Stop mirror the WinKeyer manager's control surface.
 func (k *serialKeyer) SetSpeed(wpm int) {
@@ -131,7 +143,7 @@ func (k *serialKeyer) Close() {
 
 func (k *serialKeyer) run() {
 	defer close(k.done)
-	defer k.allLow()
+	defer k.idle()
 	for {
 		select {
 		case <-k.stop:
@@ -149,7 +161,7 @@ func (k *serialKeyer) run() {
 			for {
 				select {
 				case <-k.stop:
-					k.allLow()
+					k.idle()
 					k.onBusy(false)
 					return
 				case s2 := <-k.in:

internal/winkeyer/winkeyer.go

@@ -58,6 +58,7 @@ type Config struct {
 	//                 apply; the paddle/sidetone/ratio fields are WinKeyer-only.
 	Type      string `json:"type"`
 	CWKeyLine string `json:"cw_key_line"` // serial: "dtr" (CW on DTR, PTT on RTS — default) | "rts"
+	CWInvert  bool   `json:"cw_invert"`   // serial: invert line polarity (active-LOW) for both key and PTT
 }
 
 func (c Config) normalised() Config {