build: fail early when OpenCV is older than dartcv requires - #451

Summary

When DARTCV_DISABLE_DOWNLOAD_OPENCV is set and the build is pointed at an existing OpenCV through OpenCV_DIR, find_package(OpenCV REQUIRED) accepts whatever version it finds. dartcv calls a handful of APIs added in 4.12, so an older OpenCV configures cleanly and then fails well into the build, with errors that never mention the version as the cause.

This adds a version check right after find_package, so the configure step stops with a message that names the version and the path.

How the minimum was determined

I built dartcv against several OpenCV releases, each compiled from source with core,imgproc,imgcodecs,calib3d,features2d,flann,ximgproc and contrib:

The APIs that set the floor:

So 4.12 is the constant here. The download and build-from-source paths already satisfy it — DARTCV_OPENCV_VERSION defaults to 4.12.0.0 — so nothing changes for them; this only affects builds that supply their own OpenCV.

Before and after

Building against OpenCV 4.11.0 previously produced 8 errors like:

dartcv/core/mat.cpp:553:39: error: no member named 'reinterpret' in 'cv::Mat'
dartcv/imgproc/imgproc.cpp:1061:17: error: no member named 'thresholdWithMask' in namespace 'cv'

and against 4.8.1, 31 of them, starting with a missing cv::hfloat in core/lut.hpp. Neither points at the real problem.

Now:

CMake Error at cmake/download_setup_opencv.cmake:144 (message):
  dartcv requires OpenCV 4.12 or newer, but found 4.11.0 in
  /path/to/inst-4.11.0/lib/cmake/opencv4.

Verification

• OpenCV 4.8.1 and 4.11.0 → configure stops with the message above.
• OpenCV 4.12.0 → configures and builds exactly as before; libdartcv.dylib (12 MB) with ximgproc and calib3d linked in.

Tested on macOS arm64 with each OpenCV built from source. Happy to phrase the message differently, or to move the check into CMakeLists.txt if you would rather keep download_setup_opencv.cmake to download logic only.