[CAOShurong]

Fixes #292.

On Windows, Python's filesystem converter passes UTF-8 filenames to the native
code, but narrow fopen interprets them using the ANSI codepage. This can write
to a different filename, fail for an existing Unicode parent directory, or
overwrite an unrelated ANSI-decoded filename.

Route the native file-open calls through one helper that converts valid UTF-8
paths to UTF-16 and uses _wfopen on Windows. Other platforms still call
fopen unchanged. Stream modes, serialization and Python converters are
unchanged; invalid UTF-8 is rejected rather than replaced.

The regression tests cover GDS/OAS/SVG output, streaming and raw-cell reads,
metadata, existing files moved independently with Python, str/Path/bytes
arguments, supplementary/decomposed characters and failure paths.

Validation on Windows 11, CPython 3.13.1, ANSI codepage 936, MSVC:

• Built and non-editably installed unmodified main and the patched wheels using
  the existing Windows CI configuration. The new tests fail 149 cases on main
  (52 pass); the patch passes all 201 cases repeatedly. All 91 original test
  outcomes are unchanged; the full suite passes 292 tests.
• Ran python -m pytest <checkout>/tests/file_io_test.py -q and
  python -m pytest <checkout>/tests -q from outside the source package,
  with --basetemp=<fresh-directory> after the default local temporary directory
  produced fixture permission errors.
• Built the wheel/sdist, checked source/package/installed bytes, and ran native
  C++ plus installed Python GDS/OAS/SVG success and error workflows.
• Black and changed-line C++ formatting checks pass. Whole-file clang-format
  still reports the same five pre-existing diagnostics on unmodified main.

Linux/macOS and other Python versions have not been run locally. An existing
Windows gds_timestamp(path, new_timestamp) failure also reproduces on
unmodified main and is unchanged; this patch covers timestamp reads and file
opening, not that separate timestamp-update issue.

[heitzmann]

Thanks @CAOShurong