Windows support: abstract POSIX file I/O behind a portable layer

[jose-compu]

Problem

The storage and metadata layers use POSIX-only APIs (<fcntl.h>, <unistd.h>, <sys/mman.h>, pwrite, ftruncate, fsync, mmap). LogosDB does not build on Windows.

Proposal

Port the I/O layer to Windows using CreateFileMapping / MapViewOfFile (or std::filesystem + FILE* with FlushFileBuffers). Options:

1. Introduce a small internal abstraction (src/file_io.h) with open_rw, read_at, write_at, truncate, mmap_readonly, sync. Provide POSIX and Win32 implementations behind #ifdef _WIN32.
2. Alternatively, wrap everything in std::filesystem + std::fstream and accept the performance cost on Windows only.

Also:

• Add windows-latest to the CI matrix (depends on Add GitHub Actions CI workflow (build + test on Linux/macOS) #1).
• Replace %llu printf formats with PRIu64 where needed.

Acceptance criteria

• cmake --build succeeds with MSVC on Windows.
• Test suite passes on Windows in CI.
• README build instructions mention Windows.