Fix #3139: support UTF-8 file paths on Windows via mju_fopen

[Ashutosh0x]

Problem

On Windows, MuJoCo uses fopen throughout its C/C++ layer. fopen on Windows uses the system ANSI code page, which means file paths containing UTF-8 characters (non-ASCII) fail to open even though the paths are valid.

Fixes #3139.

Changes

src/engine/engine_util_file.h (new file)

Introduces mju_fopen, a thin wrapper around fopen:

• On Windows: converts the UTF-8 filename and mode strings to wide (wchar_t) using MultiByteToWideChar(CP_UTF8, ...) and calls _wfopen. Falls back to fopen on allocation failure.
• On all other platforms: directly calls fopen with zero overhead.

The function is static inline in a header so it requires no new compile unit and has no ABI impact.

Callers updated to use mju_fopen instead of fopen:

• src/engine/engine_io.c — mj_saveModel
• src/engine/engine_print.c — mj_printFormattedModel, mj_printFormattedData, mj_printFormattedScene
• src/engine/engine_util_errmem.c — log-to-file handler, mju_writeLog
• src/user/user_api.cc — mj_encode
• src/user/user_util.cc — FileToMemory
• src/xml/xml_api.cc — mj_saveLastXML