GSoC Contributor Builds First Direct Test Suite for Astropy's C and Cython Extensions

A Google Summer of Code (GSoC) contributor spent the summer building a dedicated, low-level test suite for Astropy's compiled C and Cython extensions, which had previously been tested only indirectly through high-level Python wrappers. The lack of direct testing created a blind spot where deep regressions in C code could be masked by the Python layer above. The new test suite bypasses Astropy's public API entirely, exercising each compiled extension module directly to catch silent failures. This work serves as a critical prerequisite for three major Astropy milestones: the APE package split, a Meson build system migration, and compatibility with Python 3.15's Limited API for free-threaded builds. The contributor began with Astropy's table module, writing low-level tests for Cython join engines and handling complex edge cases including Cartesian product expansions and raw memory buffer validation.