Windows support

[tori29umai0123]

Summary

This PR adds Windows/MSVC build support for DeepGEMM while maintaining full compatibility with existing Linux/GCC
builds.

Changes

MSVC Compatibility Layer

• Add csrc/utils/msvc_compat.hpp with POSIX compatibility macros (popen, pclose, WEXITSTATUS)
• Add DG_MSVC preprocessor macro for platform-conditional compilation

Code Fixes for MSVC

• Convert C++20 designated initializers to positional brace initialization (MSVC requires aggregate types)
• Fix std::filesystem::path handling (use .string() for conversions, as c_str() returns wchar_t* on Windows)
• Add .exe suffix for Windows executables (nvcc.exe, cuobjdump.exe)

Build System

• Update setup.py with Windows-specific compiler flags (/std:c++latest, /O2, /EHsc, etc.)
• Add cuBLASLt and cuBLAS library linking
• Fix wheel platform tag (lowercase win_amd64)
• Fix wheel filename format to comply with PEP 427

Documentation

• Add Windows build instructions to README.md

Testing

• Successfully built on Windows 11 with:
  • Visual Studio 2022 (MSVC 14.43)
  • CUDA Toolkit 12.8
  • PyTorch 2.10
  • Python 3.10

Notes

• All changes use conditional compilation to preserve Linux/GCC compatibility
• No changes to kernel code or CUDA device code