Link forms, standard levels and path lengths: #641 and #642 (2026.9.15.2)

[Sunrisepeak]

Summary

The seven asks of #641 and #642, implemented in the engine as one change (2026.9.15.2). The triage record and the implementation plan are in .agents/docs/2026-09-15-641-642-*.md; every premise below was measured or read there.

• A required llvm resolves to the NDK's version on a fresh home; llvm.libcxx cannot be used by a c++20 graph; a tool's dependency-rooted source passes MAX_PATH on Windows #641 item 1. A required compiler family's version is taken from the target rows whose pin names the same payload (pinned_versions_for). A fresh home no longer resolves requires = ["mcpp:compiler=llvm"] to the Android NDK's llvm@30.0.16248370, and compiler=emsdk / compiler=android-ndk find their own pins instead of a false refusal.
• A required llvm resolves to the NDK's version on a fresh home; llvm.libcxx cannot be used by a c++20 graph; a tool's dependency-rooted source passes MAX_PATH on Windows #641 item 2. A package that provides the C++ layer and states [package] standard compiles its implementation units at exactly that level; module units, the std module included, stay at the graph's level. A c++20 program can use llvm.libcxx. The level enters the dependency cache key and the fingerprint; a provider that states nothing is byte-identical.
• A required llvm resolves to the NDK's version on a fresh home; llvm.libcxx cannot be used by a c++20 graph; a tool's dependency-rooted source passes MAX_PATH on Windows #641 item 3. A source outside its declaring package is addressed relative to its owning package (__pkg/<owner>/...) or a hashed directory (__ext/<hash>/); the host-tool sub-build scratch is <cache>/tool/.build/<hash>; ninja-invoked engine subcommands open files through an extended-length path helper on Windows.
• A required llvm resolves to the NDK's version on a fresh home; llvm.libcxx cannot be used by a c++20 graph; a tool's dependency-rooted source passes MAX_PATH on Windows #641 item 4. mcpp pack --features reaches every build pass of the pack; mcpp run --format passes its features to the pack it performs.
• A required llvm resolves to the NDK's version on a fresh home; llvm.libcxx cannot be used by a c++20 graph; a tool's dependency-rooted source passes MAX_PATH on Windows #641 item 5. A dependency's C++ shared library in a graph whose C++ runtime is a package is refused before compiling (shared-library-cxx-runtime), naming the remedies; under cxx_runtime = { shared = "self-contained" } it links a private copy of the runtime. Measured: with a private copy per image, std::runtime_error thrown in the library is not caught by that class in the program, which the refusal states.
• Explicit static linking against a package whose Android row defaults to shared: an overridable per-row form (E1) and a dependency's resolved form in build programs (E2) #642 E1. linkage = "static" | "shared" in [targets.<n>] and [target.<sel>.targets.<n>] states a package's default link form; kind = "shared" stays the constraint. The consumer's edge and a written dependency_linkage override it with an information line, which --strict accepts.
• Explicit static linking against a package whose Android row defaults to shared: an overridable per-row form (E1) and a dependency's resolved form in build programs (E2) #642 E2. Dependency link forms are computed once before the root build program and applied where they were; the root build program reads MCPP_DEP_<NAME>_LINKAGE and mcpp::dep_linkage(name).
• CI: the hermetic llvm job runs e2e 663, 690 and 696, which the shards skip, and asserts their final lines.

Closes #641
Closes #642

Test plan

• mcpp test (120 unit binaries) and mcpp test -p manifest|platform|buildmcpp|dyndep|versioning|toolchain-model pass locally (Linux x86_64)
• New e2e 689, 690, 692, 693, 696, 698 pass with the fresh binary, and each fails on the released 2026.9.15.1 for the reason it covers
• Neighbouring e2e pass: 01, 37, 55, 56, 57, 117, 124, 143, 184, 187, 193, 222, 234, 615, 638, 663, 665, 677, 678, 685, 688
• check_docs_style, check_docs_structure, check_version_pins, check_modules_wiring, check_narrow_conversions, check_matrix_reasons, check_target_tiers
• CI on Linux, macOS and Windows, including the Windows reading of e2e 698 leg C