fix(windows): use wide-char API for non-ASCII path support

[jjserenity]

Fixes #357

Replace ANSI Windows APIs with their wide-char (W) counterparts to support non-ASCII UTF-8 paths (CJK, Cyrillic, Turkish, etc.) on Windows.

Changes

• New src/foundation/win_utf8.h: UTF-8 ↔ UTF-16 conversion helpers (cbm_utf8_to_wide, cbm_wide_to_utf8)
• src/foundation/compat_fs.c:
  • cbm_opendir/cbm_readdir: FindFirstFileA/FindNextFileA → FindFirstFileW/FindNextFileW
  • cbm_mkdir_p: _mkdir → _wmkdir
  • cbm_unlink: _unlink → _wunlink
  • cbm_rmdir: _rmdir → _wrmdir
• src/foundation/platform.c:
  • cbm_mmap_read: CreateFileA/CreateFileMappingA → CreateFileW/CreateFileMappingW
  • cbm_file_exists/cbm_is_dir: GetFileAttributesA → GetFileAttributesW
  • cbm_file_size: GetFileAttributesExA → GetFileAttributesExW
• src/discover/discover.c:
  • safe_stat + try_load_nested_gitignore + cbm_discover: stat() → _wstat64() via new wide_stat() helper

Scope

Windows only — all changes are guarded by #ifdef _WIN32. POSIX paths untouched. Zero behavioral change for macOS/Linux.

Tested

• All changed files compile clean on MinGW GCC 14.2 (-Wall -Wextra -Werror -fsyntax-only)
• Full test suite requires POSIX build environment (CI will verify)

[DeusData]

Thank you, @jjserenity! 🙏 This is a clean, comprehensive wide-char migration — exactly what was needed for #357 (the Turkish-path report) and the broader Windows non-ASCII class. I reviewed it closely:

• The new win_utf8.h helpers are textbook-correct (two-call MultiByteToWideChar/WideCharToMultiByte sizing, NULL-safe, malloc'd return).
• I traced the free-discipline at every conversion site — each cbm_utf8_to_wide is freed on every return path (error and success), and the cbm_wide_to_utf8/free(u8) in readdir is balanced too. No leaks, no double-frees.
• The safe_stat→wide_stat (_wstat64), FindFirstFileW/FindNextFileW, _wmkdir/_wunlink/_wrmdir, and CreateFileW/GetFileAttributesW swaps all look right, each gated behind #ifdef _WIN32 with the POSIX path untouched.

Verified locally on macOS: the POSIX branches and shared refactors compile clean under -Werror and all 3,622 tests pass (the _WIN32 branches are validated by the Windows CI build). Merging via squash — authorship preserved. Closes #357, and contributes to the Windows cluster in #394. Thank you for making non-ASCII paths work on Windows! 🙏