feature, add OpenCV cv::Mat support for HDF5 I/O

[steven-varga]

Add h5cpp I/O integration for OpenCV cv::Mat.\n\nTask:\n- Create h5cpp/H5Mopencv.hpp (OpenCV is too large to vendor; use system find_package(OpenCV))\n - Handle RowMajor/ColMajor layouts\n - Guard non-contiguous mats (throw or copy)\n - Support cv::Mat, cv::Mat_<T>, cv::Vec, cv::Matx\n- Add example examples/linalg/opencv.cpp\n\nAcceptance Criteria:\n- [ ] cv::Mat writes to HDF5 and reads back correctly\n- [ ] Non-contiguous mats are handled gracefully