[SamiAGOURRAM]

Fixes #35.

At process exit on Linux, glibc runs the C++ static destructors (__run_exit_handlers) before the libraries' destructor functions (_dl_fini). The namespace-scope std::shared_ptr<PyState> pyState is therefore already destroyed when onLibraryUnload → finalizePythonInterpreter assigns it nullptr, and that assignment decrements a reference count in freed memory.

This change keeps the state and its mutex in objects that are allocated once and never destroyed (function-local static pointers), so finalizePythonInterpreter always resets a live shared_ptr; it also takes the mutex, as instantiation does. Nothing else changes; the Windows DllMain path is untouched.

Checked:

• valgrind on a one-output FMU (reproduction in Use-after-free at process exit on Linux: finalizePythonInterpreter resets a shared_ptr already destroyed by static destructors (heap corruption, "corrupted double-linked list") #35): 2 invalid reads in finalizePythonInterpreter with 0.3.4's library, 0 with this change.
• An FMU whose slave imports SciPy: aborts at exit ("corrupted double-linked list") in 7 of 12 runs with 0.3.4's library, 0 of 12 with this change.
• These runs used 0.3.4's source with the same diff; on this branch (master + this commit), pythonfmu-export builds on Linux (python:3.11-slim, g++ 14, cmake Release). Not built on Windows or macOS.

[StephenSmith25]

I'll look at this by tomorrow but thanks for reporting the issue and fixing it!