Document Fortran ABI assumptions

[krystophny]

Summary

• document when non-BIND(C) Fortran calls are acceptable and when BIND(C) should be preferred
• preserve old lowercase fortran_module / fortran_library imports used by existing tests
• make the parser compatible with current textX class-name validation
• cache NumPy array descriptors and isolate tracemalloc memory tests

Fixes #1

Verification

Test fails on main

$ python -m pytest -q
ModuleNotFoundError: No module named 'textx'

After installing python-textx from AUR, main also hit stale API/parser compatibility failures before this patch.

Test passes after fix

$ python -m pytest -q
22 passed, 23 warnings in 3.43s