feat: a build program declares a runtime library directory; a passing check moves its stamp #701

Motivation

A build program can link a prebuilt library (link_flag, link_search) and place a file beside the program (deploy), but it cannot say where the program's shared libraries are found at run time. The manifest key [runtime] library_dirs does exactly that — it reaches mcpp run's loader path and mcpp pack's closure search — but it is a fixed TOML array, and the directories in question are known only to the build program: a vcpkg prefix's bin/, a Qt SDK's bin/ resolved through mcpp::xpkg_dir.

Measured against the engine: mcpp pack's PE closure searches plan.runtimeLibraryDirs only (src/pack/pipeline.cppm), which is fed from [runtime] library_dirs and never from link_search/link_flag (src/build/plan.cppm). A plugin that installs a vcpkg prefix therefore has no way to make its DLLs reach mcpp run or the packed tree except one mcpp::deploy per file — and on a project's first build the files do not exist yet when the build program runs.

Found while implementing the same plugins: a role = "check" action whose command writes no stamp (the engine writes it, 2026.8.29.1+) re-runs on every build after one of its inputs changes. __action-stamp only creates a missing stamp and leaves an existing one alone, so after the input changes and the check passes again, the stamp stays older than the input forever.

Design

1. mcpp::runtime_library_dir(const char* dir), wire mcpp:runtime-library-dir=<dir>, protocol 12: the build-program form of [runtime] library_dirs. Transform::AbsPath, Scope::LinkGlobal, persisted in the build-program cache. apply() folds it into RuntimeConfig::libraryDirs — the field the manifest key fills — so run, pack and the ELF/Mach-O rpath rendering treat it as they treat the manifest key. The root package's own directive is mirrored into the plan snapshot the way deploy already is.
2. __action-stamp records each stamp's modification time before the command runs; on success it creates a missing stamp and moves an existing stamp it did not write to the present. A stamp the command created or rewrote is left alone, so wrapper scripts keep working.

Scope

src/build/hostprogram.cppm, modules/buildmcpp/src/{directives,program_protocol}.cppm, src/build/prepare.cppm, src/cli.cppm, docs/04 and docs/30 (en, zh), unit tests, e2e 779 and 780.

Consumer: mcpp-community/mcpp-plugins 0.13.0 (deps-vcpkg, deps-cmake, rules-qt).