[Boulea7]

Parsing U"ab" with char_ >> char_ into a std::u32string succeeds but leaves only U"b": each character uses scalar assignment and replaces the previous contents. Extend the existing std::string exception to std::u32string and std::u8string so these elements are appended, including UTF-32-to-UTF-8 transcoding.

The regression covers narrow and UTF-32 input, non-BMP code points, and UTF-8 output. Direct builds of github_issues.cpp pass with GCC 12 (C++17/20) and Clang 14/19 (C++20); GCC checks for sequence attributes, parser attributes, and merging also pass. These local checks use Boost 1.74 support headers.

CI is incomplete. GitHub Actions awaits maintainer approval; all five workflows have zero jobs. Drone finished with 21 successful, 10 failed and four terminated stages. Its Clang 14/C++20 compile_all_t failure reproduces identically on unmodified develop at 293f5e1; other stages report related all_t/ranges diagnostics (see #323). The Xcode 14.0 runner exits with an unaccepted Xcode license. The Xcode 15.0.1 failure and terminated stages have not been fully diagnosed.

Fixes #327.

[tzlaine]

Thanks! I had to resolve a merge conflict, and did it locally and then pushed that. So I'm closing this PR without a merge.