fix(mysql-connector-cpp): state the C++ standard library as a layer requirement

[Sunrisepeak]

compat.mysql-connector-cpp builds static libraries with CMake and each platform's system compiler: g++ with libstdc++ on Linux, Apple clang with libc++ on macOS. On Linux, a consumer whose toolchain links libc++ failed at link:

ld.lld: error: undefined symbol: std::__cxx11::basic_string<...>::_M_create(...)

The index CI keeps this member off the llvm leg for that reason. The skip's comment deferred to mcpp-community/mcpp#613, on the expectation that the install hook would one day see the consumer's standard library.

What mcpp#613 established

The hook cannot see it, and not for lack of a variable. mcpp resolves the toolchain after the dependency graph, because a package in the graph may supply a target-side layer. When a dependency's install hook runs, no standard library has been decided yet.

mcpp 2026.9.12.2 emits MCPP_CXX_STDLIB to install hooks, and it is empty there. mcpp's tests/e2e/648 measures this.

The requirement is therefore stated in the descriptor, with the layer grammar mcpp already enforces.

Change

• The descriptor states the requirement in its per-platform tables, which the reader splices into the mcpp segment for the target platform: requires = { "mcpp:c++-abi=libstdc++" } under linux, and requires = { "mcpp:c++-abi=libc++" } under macosx. A consumer whose toolchain resolves the other implementation is refused at resolution, naming both, instead of failing at link.
• The first revision stated libstdc++ for every platform, and the macOS default leg of this PR's CI refused the member, which builds and links there. That leg is the measurement for the macosx entry.
• The comment in install() states why the hook cannot adapt. The probe's expected value becomes "nil or empty".
• The llvm-leg skip in validate.yml stays, with the reason updated. The refusal follows the hook's source build (about 881 s), so the leg would spend the build to reach a known refusal.

Measurements

The descriptor form of the requirement, scoped the same way (linux = { requires = ... }, macosx = { requires = ... }), was measured with mcpp 2026.9.12.2 on a local index package that has no download:

$ mcpp build --toolchain llvm@22.1.8      # exit 2
error: `stdreqp@1.0.0` requires the c++-abi to be `libstdc++`.
         c++-abi           libc++         (payload)
         required          libstdc++      (required by stdreqp@1.0.0)

$ mcpp build                              # gcc@16.1.0, exit 0

Compatibility with the index floor (min_mcpp = 2026.8.27.2):

• Before 2026.8.29.1: descriptor requires is not read. That reader records the key as unknown and prints a warning (warn_unknown_xpkg_keys), then proceeds as before, so the descriptor does not raise the floor.
• From 2026.8.29.1: the key and the c++-abi layer are read and enforced.

The lint loop from validate.yml passes locally on the file (Lua syntax, mirror URLs, package name), and mcpp xpkg parse --all-os, which refuses unknown keys, accepts both the linux and the macosx sections.