fix: avoid DTR/RTS pulse on serial port open

Previously SetCommState was called with fDtrControl and
fRtsControl left at whatever GetCommState returned (often
enabled), then EscapeCommFunction lowered DTR/RTS after the
fact. This produced a brief low-high-low pulse on connect,
which can reset USB-UART-connected devices.

Set DTR_CONTROL_DISABLE and RTS_CONTROL_DISABLE directly in
the DCB before the single SetCommState call, so the lines
never get pulsed.

celrs/platform/serial_win.c

@@ -27,6 +27,13 @@ cel_serial_platform_handle cel_serial_platform_open(char const* path, int baud_r
     dcb.Parity   = NOPARITY;
     dcb.StopBits = ONESTOPBIT;
 
+    /* Keep DTR/RTS low so the TX module is not reset on connect. Setting
+     * these in the same SetCommState call (rather than via a later
+     * EscapeCommFunction) avoids a brief DTR/RTS-high pulse if the port's
+     * existing DCB has them enabled. */
+    dcb.fDtrControl = DTR_CONTROL_DISABLE;
+    dcb.fRtsControl = RTS_CONTROL_DISABLE;
+
     if (!SetCommState(h, &dcb)) {
         CloseHandle(h);
         return CEL_SERIAL_PLATFORM_INVALID_HANDLE;