macOS build failure with libc++ 19

[AndriiVoitenko]

libMVRgdtf fails to build on macOS with Homebrew LLVM 19:

error: implicit instantiation of undefined template 'std::char_traits'

On macOS, TXChar is UniChar (uint16_t), so StdUStr becomes std::basic_string<uint16_t>. libc++ 19 no longer provides std::char_traits for arbitrary types — only char, wchar_t, char8_t, char16_t, and char32_t.

Linux is already fine (TXChar = char16_t). The fix is to do the same on macOS in GSString.h:55:
// Before:
typedef UniChar TXChar;
// After:
typedef char16_t TXChar;

This is safe because:

• The public API only exposes MvrString (const char*) — no type change visible to consumers
• char16_t and uint16_t are the same width and representation
• It actually fixes a latent UB in TXString::hash(), which already casts to std::u16string
• CoreFoundation calls expecting UniChar* just need an explicit cast