fix #381

[foryoung365]

std::fstream fs(simplePath, std::ios::app);

File existence check using fstream with ios::app is implemented as a workaround because:

• ifstream::good() incorrectly handles directories on Unix-like systems
• <sys/stat.h> adds platform-specific dependencies
• It avoids duplication with CppCheck's 'path' implementation
• std::filesystem (the proper solution) requires C++17 support

[firewave]

Thanks. Please give me a few days since I might be out sick.