fix: serial CW keyer PTT dropping between words on CP210x (SCU-17)

Confirmed from HB9HBY's log: ptt=true, yet the FT-891 dropped to RX between
words during a CQ macro — while the same SCU-17 works in N1MM. Root cause:
go.bug.st/serial drives DTR/RTS through GetCommState/SetCommState, which on the
SCU-17's Silicon Labs CP210x drops the asserted RTS (PTT) the moment DTR (CW)
is toggled. N1MM/WSJT use EscapeCommFunction, which sets one line without
disturbing the other.

OpsLog now drives the lines the same way: a new lineCtl abstraction with a
Windows implementation (linectl_windows.go) that opens the COM port via
CreateFile and toggles DTR/RTS with windows.EscapeCommFunction (SETDTR/CLRDTR/
SETRTS/CLRRTS). linectl_other.go keeps go.bug.st/serial for non-Windows builds.
The serial keyer holds a lineCtl instead of a serial.Port. RTS (PTT) now stays
asserted for the whole macro, so the rig holds TX between words.

Promotes golang.org/x/sys to a direct dependency.

internal/winkeyer/linectl_other.go

@@ -0,0 +1,37 @@
+//go:build !windows
+
+package winkeyer
+
+// linectl_other.go — non-Windows DTR/RTS line control via go.bug.st/serial. On
+// Linux/macOS the SetDTR/SetRTS ioctls control the lines independently, so the
+// CP210x-on-Windows problem that motivated the native path (see linectl_windows.go)
+// doesn't apply. OpsLog ships on Windows; this keeps the package building elsewhere.
+
+import (
+	"fmt"
+	"strings"
+
+	"go.bug.st/serial"
+)
+
+type serialLineCtl struct {
+	port serial.Port
+}
+
+func openLineCtl(port string) (lineCtl, error) {
+	if strings.TrimSpace(port) == "" {
+		return nil, fmt.Errorf("no serial port")
+	}
+	p, err := serial.Open(port, &serial.Mode{BaudRate: 9600, DataBits: 8, Parity: serial.NoParity, StopBits: serial.OneStopBit})
+	if err != nil {
+		return nil, fmt.Errorf("open %s: %w", port, err)
+	}
+	c := &serialLineCtl{port: p}
+	_ = c.setDTR(false)
+	_ = c.setRTS(false)
+	return c, nil
+}
+
+func (c *serialLineCtl) setDTR(on bool) error { return c.port.SetDTR(on) }
+func (c *serialLineCtl) setRTS(on bool) error { return c.port.SetRTS(on) }
+func (c *serialLineCtl) close() error         { return c.port.Close() }