Fix Windows build break in hdf5_utils.cpp by making access() include cross-platform

[Copilot]

Windows CI fails in src/hdf5/hdf5_utils.cpp because <unistd.h> is POSIX-only and unavailable under MSVC. The file uses access(path, W_OK), so the include and symbols must be mapped to Windows equivalents while preserving current POSIX behavior.

• What changed

  • Replaced the unconditional POSIX include with a platform-gated include block in src/hdf5/hdf5_utils.cpp.
  • On Windows (_WIN32), include <io.h> and map:
    • access → _access
    • W_OK → 2
  • On non-Windows, continue using <unistd.h> unchanged.

• Scope

  • No behavioral or logic changes outside this include/macro compatibility block.
  • No changes to errno handling (EAGAIN, EBUSY), which remains provided by <errno.h>.

#ifdef _WIN32
#  include <io.h>       // _access()
#  define access  _access
#  define W_OK    2
#else
#  include <unistd.h>
#endif

[olivhoenen]

Added directly in #69 to allow relevant tests