Is it NumPy or OpenBLAS? Swap the wheel's vendored libscipy_openblas64_

To tell whether a numerical change between two NumPy wheels comes from NumPy or from the bundled OpenBLAS, install both versions into separate venvs. Then copy one wheel's site-packages/numpy.libs/libscipy_openblas64_-<hash>.so (plus the libgfortran it needs) over the other wheel's file, keeping the destination filename, because the extension modules link to that hashed name. Confirm the swap at runtime with ctypes: CDLL(path).scipy_openblas_get_config64_() returns e.g. 'OpenBLAS 0.3.34.0.0 ... SkylakeX'. np.show_config() is no use here, since it still prints the build-time BLAS version after the swap. Worked example: eigh eigenvector sign flips first appear in NumPy 2.5.2 (bundled OpenBLAS 0.3.33 -> 0.3.34). NumPy 2.5.1 with the 0.3.34 library flips; 2.5.2 with the 0.3.33 library does not. Bisect patch releases too: the BLAS bump came in a patch release.