Implement wide-character and multibyte string support

[pbalduino]

Goal

Add wide-character (wchar_t) and multibyte string handling for Unicode support.

Context

No wide-character support exists. This limits Unicode handling and international text.

Definition of Done

• [ ] wchar_t type and <wchar.h> header
• [ ] mbstate_t for conversion state
• [ ] Wide string functions (wcslen, wcscpy, wcscmp, etc.)
• [ ] Wide I/O (wprintf, wscanf, fwide, etc.)
• [ ] Multibyte conversion (mbstowcs, wcstombs, mbtowc, wctomb)
• [ ] UTF-8 encoding support
• [ ] mbrtowc() / wcrtomb() - Restartable conversions

Dependencies

Required

• #193 - Minimal libc ✅

Priority

Low - Nice to have

Estimated Time

10-12 weeks part-time