Fix UTF-8 file path handling on windows

[mar-yan24]

Summary

Fixes #3139: MuJoCo fails to load XML files from directories containing non-ASCII characters (e.g., Japanese, Chinese) on Windows.

The cause of the issue was that on Windows, fopen() and _stat() default to ANSI encoding, not UTF-8. Paths with non-ASCII characters get jumbled, producing errors like: ParseXML: Error opening file 'C\temp\UTF-8(randomchars)\cube\cube_3x3x3.xml'

I fixed this using a cross-platform wrapper mju_fopen() that converts UTF-8 paths to wide strings via MultiByteToWideChar and calls _wfopen on Windows, passing through to fopen on other platforms. Similarly fixed stat → _wstat in the VFS layer.

All file-opening call sites are updated:

• engine_util_errmem.c/h — mju_fopen() implementation and declaration
• engine_io.c — mj_saveModel
• engine_print.c — mj_printFormattedModel/Data/Scene
• user_util.cc — FileToMemory (key XML loading path)
• user_vfs.cc — OpenFile (_stat → _wstat wrapper)
• xml_api.cc — mj_saveLastXML, mj_printSchema, mj_saveXML
• xml_util.cc — mju_getXMLDependencies

Test plan

In line with best contribution practices, I also wrote a short testcase for mju_fopen in engine_util_errmem_test.cc.

These tests consist of

• ASCII path round-trip (write + read)
• Nonexistent file returns NULL
• UTF-8 path round-trip with Japanese characters (gracefully skips if the filesystem doesn't support UTF-8)

All 8 tests in engine_util_errmem_test passed.