Fix find of an empty substring at the end of a string_view

[fhgffy]

Summary

An empty substring should match at pos whenever pos <= size(). The nonstd implementation currently returns npos at pos == size(), including when both views are empty.

Handle this valid empty match before the existing boundary check and search-result conversion. The existing nonempty search path and rejection of positions beyond the end remain unchanged.

Add regression coverage for the view, C-string and counted-pointer overloads; default/literal empty views; interior, end, beyond-end and npos positions; nonempty controls; wide characters; and C++14-and-later constant evaluation.

Validation

Tested locally on Linux x86_64 with GCC 14.2.0:

• New tests fail on upstream 2b39e871c60f615e91f27b52881d9552ce4d0a6c: runtime failures in C++98/11 and four failing constant-expression assertions in C++14/17/20
• Patched upstream suite: all 93 cases pass in C++98/11/14/17/20 at both -O0 and -O2, using the default search configuration
• Official CMake Release build, including examples, and all four CTests pass
• Direct comparison with std::basic_string_view: all 384 checks pass for each C++17/20 × -O0/-O2 run, covering char, wchar_t, char16_t and char32_t

Clang, MSVC, older compiler versions and remote CI have not been run locally.