Add API audit tests for bugs 64-80

Introduce a comprehensive MSTest file adding unit tests for API correctness bugs (64–80) discovered during a memory-safety audit. Tests assert NumPy 2.x behavior and cover issues such as dtype preservation in np.sign, NDArray comparison/operator overloads, boolean mask and fancy-indexing getter/setter failures, scalar casting reinterpreting bytes, reshape-to-scalar handling, argmin/NaN semantics, boolean reductions (prod/cumsum), sign(NaN) crash, std/var on empty arrays, and Python-style modulo semantics. Each test documents the expected NumPy behavior and will fail until the corresponding bug is fixed.

Author: Nucs