0.15.0: mechanisms in features, build.mcpp-first configuration, the runtime closure in the payload, deps without mcpp-deps, Linux C++ library alignment

[speak-agent]

mcpp is a general engine with a framework for build plugins. Plugins come from this package, from third parties, or from the project itself, and build.mcpp is where a project uses them. 0.15.0 brings the collection in line with that design. The architecture and ecosystem plan is .agents/docs/2026-09-26-plugins-0.15-architecture-plan.md, and the conventions are docs/plugin-development.md (new).

Features state mechanisms. A feature declares only the tools its own actions run.

• rules-qt-xim, rules-qt-xim-base and rules-qt-xim-addons are removed. They pinned Qt 6.11.1 and, on Linux, glib, zstd and zlib.
• rules-qt finds the SDK from options::root in build.mcpp, then QT_ROOT_DIR, then a Qt payload the project declares in its own [xlings] table at the version it chooses. It records the level that decided as the fact rules-qt.sdk.
• The runtime closure is now the payload's responsibility, on every platform (feat: qt and qt-base carry their runtime closure (Linux: libdbus, xcb, ... under the xlings loader; Windows: the VC++ runtime) openxlings/xim-pkgindex#884, #885):
  • Linux: libdbus, fontconfig, xcb and the other libraries Qt loads;
  • Windows: the VC++ runtime.
• On Linux, rules-qt warns under a libc++ toolchain, because Qt's official build uses libstdc++.

No helper program. The deps actions run the installer directly:

• vcpkg install, configured through its own arguments: --vcpkg-root, --disable-metrics, short build and package trees, and vcpkg's own install-root lock;
• cmake -P over a script the member writes, which configures, builds and installs, or empties and extracts.

tools/deps_main.cpp and the mcpp-deps target are removed, so consumers drop tools = ["mcpp-deps"] from the edge.

Dependencies built with the program's C++ standard library on Linux.

• Under a libc++ toolchain, deps-vcpkg generates <arch>-linux-libcxx and builds ports with mcpp's clang.
• Each triplet is its own vcpkg installation, with prefix <install root>/<triplet>/<triplet>.
• deps-cmake passes mcpp's clang.

Docs. The README is now about 100 lines, down from 1113. Per-member detail moved to docs/<member>.md, and docs/plugin-development.md is new.

Breaking changes. Each takes one or two lines to migrate:

• rules-qt-xim* → rules-qt plus "xim:qt-base" = "<version>" in [xlings];
• delete tools = ["mcpp-deps"] from the edge;
• use the returned prefix instead of a hard-coded vcpkg_installed/<triplet>.

CI.

• New criteria: qt-sdk-consumer (all three SDK levels), a Linux Widgets run and pack that checks libdbus, xkbcommon and fontconfig in the packed tree, and the VC++ runtime beside the program and in the packed tree on Windows.
• The Linux Widgets and Windows CRT criteria need xim-pkgindex #885 and #884 published first.
• Locally, the vcpkg-consumer, cmake-consumer, archive-consumer, vcpkg-workspace and vcpkg-libcxx criteria pass without mcpp-deps.
• The SDK levels (options, QT_ROOT_DIR, payload) were checked locally.