feat: Flex CWX type-ahead backspace (cwx erase) — phase 2

Completes the CWX type-ahead loop: with send-on-type the keyer-panel CW text
already streams each typed char to the radio's CWX buffer (which keys in order,
so you can keep typing while it sends); this adds the matching un-send. Route
wkBackspace to FlexBackspaceCW -> cwx erase N, so backspacing a mistyped char in
send-on-type removes it from the buffer before the radio keys it.

internal/cat/flex.go

@@ -1601,6 +1601,17 @@ func (f *Flex) StopCW() error {
 	return nil
 }
 
+// BackspaceCW removes the last n not-yet-keyed characters from the CWX buffer —
+// the "un-send while sending" a serial WinKeyer can't do. Used for type-ahead
+// corrections (backspacing a mistyped call before the radio has keyed it).
+func (f *Flex) BackspaceCW(n int) error {
+	if n < 1 {
+		n = 1
+	}
+	f.send(fmt.Sprintf("cwx erase %d", n))
+	return nil
+}
+
 func (f *Flex) SetCWSidetone(on bool) error {
 	f.mu.Lock()
 	f.tx.cwSidetone = on