Add Windows wrapper for GD32 flashing

Add a `flash.cmd` helper for Windows that locates Python 3, creates a dedicated virtual environment if needed, installs `pyserial` on demand, and then runs `flash.py`. This makes the GD32 flashing workflow easier to run on Windows without manual environment setup.

Author: vanvught

common/scripts/gd32/flash.cmd

@@ -0,0 +1,55 @@
+@echo off
+setlocal EnableExtensions
+
+rem Directory containing this .cmd file
+set "DIR=%~dp0"
+
+rem Prefer the Windows Python launcher, then fall back to python.exe
+set "PYTHON_CMD="
+where py >nul 2>&1
+if not errorlevel 1 (
+    set "PYTHON_CMD=py -3"
+) else (
+    where python >nul 2>&1
+    if not errorlevel 1 set "PYTHON_CMD=python"
+)
+
+if not defined PYTHON_CMD (
+    echo ERROR: No Python 3 installation found. 1>&2
+    exit /b 1
+)
+
+set "VENV=%USERPROFILE%\venvs\gd32-common"
+set "PY=%VENV%\Scripts\python.exe"
+
+if not exist "%PY%" (
+    echo Creating virtual environment: %VENV%
+    %PYTHON_CMD% -m venv "%VENV%"
+    if errorlevel 1 (
+        echo ERROR: Failed to create virtual environment. 1>&2
+        exit /b 1
+    )
+)
+
+rem Install pyserial only when it is missing.
+"%PY%" -c "import serial" >nul 2>&1
+if errorlevel 1 (
+    echo Installing pyserial...
+    "%PY%" -m pip install -U pip
+    if errorlevel 1 exit /b 1
+
+    "%PY%" -m pip install -U pyserial
+    if errorlevel 1 exit /b 1
+)
+
+rem Final dependency check.
+"%PY%" -c "import serial; print('PySerial OK')"
+if errorlevel 1 (
+    echo ERROR: PySerial is not working. 1>&2
+    exit /b 1
+)
+
+"%PY%" "%DIR%flash.py" %*
+set "RC=%ERRORLEVEL%"
+
+endlocal & exit /b %RC%