Update Windows build documentation

build_wsl_windows.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if ! command -v x86_64-w64-mingw32-g++ >/dev/null 2>&1; then
+    echo "Missing x86_64-w64-mingw32-g++."
+    echo "Install it in WSL with: sudo apt install mingw-w64"
+    exit 1
+fi
+
+if [[ "${QT_MINGW_PREFIX:-}" == "" ]]; then
+    echo "Missing QT_MINGW_PREFIX."
+    echo "Example: export QT_MINGW_PREFIX=/opt/mxe/usr/x86_64-w64-mingw32.shared/qt6"
+    exit 1
+fi
+
+if [[ "${OpenCV_DIR:-}" == "" ]]; then
+    echo "Missing OpenCV_DIR."
+    echo "Example: export OpenCV_DIR=/opt/mxe/usr/x86_64-w64-mingw32.shared/lib/cmake/opencv4"
+    exit 1
+fi
+
+cmake -S . -B build-wsl-windows \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/mingw64.cmake \
+    -DCMAKE_PREFIX_PATH="${QT_MINGW_PREFIX}" \
+    -DOpenCV_DIR="${OpenCV_DIR}"
+
+cmake --build build-wsl-windows --config Release
+
+echo "Windows executable should be under build-wsl-windows/."