fix: replace FILE with std::fstreams

[lou-16]

Right now we’re using FILE* for file I/O, which works but is a C-style approach. For modern C++20 code, it makes sense to move to std::fstream because it’s safer and more idiomatic.

What needs to be done

• Replace FILE* usage with std::ifstream / std::ofstream where applicable.
• Remove FILE* usage from the codebase to reduce unsafe manual management.

Why

• Safer file handling (automatic closing, exception-safe).
• Cleaner, modern C++ style.
• Reduces chances of leaks or undefined behavior from forgetting fclose.

Things to check

• Make sure nothing breaks for modules still relying on C-style I/O.