Replace wstring_convert with the Win32 conversion functions

[mnightingale]

<codecvt> and std::wstring_convert were deprecated in C++17 and are removed in C++26, so utf8.cpp has to move off them eventually. This replaces them with MultiByteToWideChar/WideCharToMultiByte.

MB_ERR_INVALID_CHARS and WC_ERR_INVALID_CHARS are not new strictness, they preserve the old behaviour.
wstring_convert threw std::range_error on ill-formed input and the code caught it; without those flags the replacement would silently substitute U+FFFD instead.
The conversions now return bool and report failure to the caller rather than logging and returning an empty string.

Two bugs fixed:

• The long-path prefix for UNC paths doubled the separator, producing \\?\UNC\\server\share
• WideToUtf8ArgsAdapter mishandled a null in wargv: --m_argc; --i; continue; re-examines the same index while shrinking the count, so one null argument dropped every argument

utf8.{h,cpp} is now #ifdef _WIN32, since it is Win32-only; the unit test exits 77 (SKIP) elsewhere.