fix: implement MappedFile on POSIX (mmap)

[oozimok]

Summary

Adds a POSIX implementation of MappedFile (macOS, Linux, etc.) using open / fstat / mmap, so PE scanning is not Windows-only at compile time.

Problem

The class and its implementation were guarded with #ifdef _WIN32. On macOS the header did not declare the type and mapped_file.cpp compiled to an empty translation unit, so pe_scanner.cpp failed to build (unknown type name 'MappedFile'), and the linker reported no symbols for mapped_file.cpp.o.

Solution

• Declare MappedFile on all platforms, with separate private members for Windows vs POSIX.
• On non-Windows: open the file, ensure it is a regular file, map it read-only with mmap, and unmap/close in the destructor.
• Zero-length files: skip mmap; object remains valid and get_data() returns an empty std::span.

Testing

• cmake --build … --config Release on macOS — full build including keydot succeeds.

Notes

• Public API (is_valid(), get_data()) matches the Windows behavior; constructor failures log to stderr, consistent with the Win32 path.

[Titoot]

one thing to note, The utils.h was for some reason included twice which is unnecessary, you can just put the import outside of the _WIN32

and also do you suggest editing the build workflow to include other platforms (MacOS, Linux)?

[Titoot]

@oozimok