[pogoflow]

Since 26e75c4 (#282) the mod runtime hands \\?\ paths to std::filesystem. With the MSYS2 mingw64 toolchain from the README (GCC 16.2 here), libstdc++'s status calls go through the CRT's stat, which doesn't accept extended paths:

call on a \\?\ path	result on mingw64
create_directories	EINVAL, also when the directory exists
is_directory, is_regular_file, exists	false (ENOENT)
remove	false, no error, nothing removed
create_directory, rename, ifstream/ofstream	work

So on Windows scan() stops at "cannot create mods directory: Invalid argument", the host prints mods: unavailable, and run_c_tests.sh fails in mod_presentation_only_test. long_path_test fails in its own fixture setup (create_directories(fixture_io(...))).

Change

• ModPaths gets kind, exists, is_directory, is_regular_file, is_symlink, create_directories and remove. On Windows they use GetFileAttributesW, CreateDirectoryW, DeleteFileW and RemoveDirectoryW on the extended path; elsewhere they're the std::filesystem calls. create_directories walks the plain absolute path and creates each missing level through its extended form.
• mod_runtime.cpp uses them where it passed for_io(...) to a status call, create_directories or remove. rename and the streams stay as they are.
• long_path_test uses them for its fixture (creating directories, checking, the payload removal and cleanup).
• The relative-input case now runs from the fixture root. above.lexically_relative(fs::current_path()) returned an empty path when the caller's directory spelled the drive letter c: and the fixture C: (Windows keeps whatever spelling the caller used), so the case tested an empty path, and whether it passed depended on the caller's directory.

Testing

On Windows 11, MSYS2 mingw64 GCC 16.2:

• Before: mod_presentation_only_test fails with "cannot create mods directory: Invalid argument"; long_path_test fails with "filesystem error: cannot create directories: Invalid argument".
• After: both pass, long_path_test with the working directory spelled C:\… and c:\….

I haven't built it with MSVC or on Linux/macOS. Off Windows the helpers are the plain std::filesystem calls.