Conda forge blas

[Transurgeon]

Closes #126. With this change, the conda-forge recipe (conda-forge/staged-recipes#34978) can drop 0001-use-conda-forge-blas.patch once a sparsediffpy release ships with it.

Changes (CMakeLists.txt)

• MSVC BLAS: find_package(OpenBLAS CONFIG QUIET). If that provides the OpenBLAS::OpenBLAS target (vcpkg), link it. If not, fall back to find_library(openblas) plus find_path(cblas.h), which is how conda-forge's openblas is laid out. The check is on TARGET rather than OpenBLAS_FOUND, because OpenBLAS's own config file doesn't define the imported target.
• Unix BLAS: after find_package(BLAS), link libcblas if it sits in the same directory as the BLAS that was found, and link it before BLAS. On conda-forge, libblas.so points to libopenblas at build time, so a cblas_ddot link probe would pass and libcblas would be skipped. The package would then break once a user switches to netlib BLAS. Linking libcblas explicitly is what conda-forge expects. On conda-forge this links the same libraries as the recipe's current patch, which has passed staged-recipes CI. The difference is that the patch accepts a libcblas from anywhere on the search path, while this only looks next to the BLAS that was found.
• SDE_WARNINGS_AS_ERRORS: gates MSVC /WX. It defaults to ON for standalone builds (engine CI is unchanged) and OFF under scikit-build, so downstream MSVC toolchains don't break on new warnings. SparseDiffPy's wheel CI can pass -DSDE_WARNINGS_AS_ERRORS=ON to keep /WX.

Unchanged

• vcpkg (engine CI, Windows wheels) still takes the config path.
• Ubuntu libopenblas-dev: linking is the same as before, unless a separate libcblas (e.g. from ATLAS) is installed in the same directory. In that case it also gets linked, which is harmless.
• macOS Accelerate is untouched.

Testing

• macOS: standalone build plus ctest pass.
• Ubuntu and Windows (vcpkg): covered by engine CI.
• conda-forge paths: there's a new conda-forge BLAS workflow (.github/workflows/conda-forge.yml). It builds and runs ctest against conda-forge packages installed with Miniforge on Linux (libblas/libcblas, BLA_VENDOR=Generic) and on Windows (openblas, no CMake config). On Linux it also checks that libcblas.so is a NEEDED entry. It then switches to netlib libblas/libcblas and runs the tests again, which only passes because libcblas is linked explicitly.

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