0.19.1: json::value compiles under clang with libstdc++, and CI reads every build program as an editor does

[speak-agent]

Closes #46.

The defect

mcpp::plugins::json::value held std::vector<std::pair<std::string, value>> members. Under clang with libstdc++'s std module the pair is instantiated while value is incomplete (incomplete type 'std::pair<…, value>' used in type trait expression), so mcpp.plugins does not compile. That is the combination an editor's clangd uses to read a project built with GCC, so every import mcpp.…; in such a project's build.mcpp showed Module file '…/mcpp.rules.qt.pcm' not found. GCC and clang with libc++ accept the code; no build failed.

The fix

struct member { std::string first; value second; }, with get() defined after both types are complete. members is used only in src/plugins.cppm, and emplace_back(key, v) builds the aggregate through C++20's parenthesized aggregate initialisation, so nothing else changes.

CI: the two combinations nothing compiled

job	compiler + library	what it runs
consumers (unchanged)	GCC 16 + libstdc++	every fixture
toolchain-llvm (new)	llvm@22.1.8 + libc++	builds and runs all-rules-compile (every member, called), plugin-logic (29 cases), embed ×3, island-interface, archive-consumer, spirv-module-consumer; --configure-only for the four Qt fixtures, whose Qt binaries are libstdc++ — the build program and library still compile and run
toolchain-editor (new)	mcppls 0.0.15's clangd + libstdc++	mcppls diagnostics build.mcpp on all 37 fixtures with a build program; fails on an error, on a file it did not read, or on a run with no report; every diagnostic goes to the job summary

The device-toolchain fixtures (cuda, hip, sycl, multi-rule) already build with llvm@22.1.8 in consumers, and the deps members' libc++ rows are vcpkg-libcxx and cmake-cache there, so the llvm lane does not repeat them.

Measured locally before pushing:

• the editor lane on the previous src/plugins.cppm: 34 of 37 fixtures fail with the error above; with this change, 0 errors in all 37;
• the llvm lane: every step passes; the lane also asserts the build was linked with -stdlib=libc++.

The rule

docs/plugin-development.md §4: a plugin is the C++23 that all three accept (MSVC as well for the members that claim Windows), with the differences met so far — no std::pair/std::tuple element for a type holding a container of itself, no compiler's leniency, no std::println in a build program (macOS 14), no std::filesystem::path iterator in a module (MSVC 14.52).

Version 0.19.1: source-compatible, package floor unchanged (mcpp 2026.10.1.3).