[Sunrisepeak]

Summary

Closes #833. Fixes the mcpp half of #832 (the index half is openxlings/xim-pkgindex#1032).

The target base. A sysroot is the prebuilt supplier of a target's lower layers, not a layer. [target.<triple>].sysroot takes a package, a locator (glibc@system, musl@system, macosx|iphoneos|iphonesimulator@system|<version>, msvc@…), { path = "<dir>", provides = [...] } or "", from four tiers (--sysroot / MCPP_TARGET_<TRIPLE>_SYSROOT / MCPP_SYSROOT → the project → config.toml → the target row). It is resolved once into tc.base (mcpp.toolchain.base::Resolution) — the five groups of base fields Toolchain carried move there — and every reader uses it.

• Measured, not guessed: the driver says where the C runtime is, the preprocessor which C library and version. Debian multiarch is supplied only to a gcc that needs it (glibc@system builds with gcc and clang). A base older than the compiler's runtime requires is refused before compiling, naming both versions.
• The artifact binds its base: PT_INTERP and RUNPATH name the base's loader and C library; rule B checks against it; the base's identity enters the fingerprint; mcpp pack states the glibc floor in HOST-REQUIREMENTS.
• Fixed: a stated base hands ld --sysroot (gcc's clean link specs drop the driver's); no artifact loads the host's loader over a sysroot's libc; mcpp no longer writes into a vendor's sysroot; a toolchain phase's spec + path-only keys is refused, spec + sysroot is the program's statement of the base.
• Apple and MSVC: SDKROOT names an SDK on every host; an SDK without the row's slice is refused naming it; x86_64-ios-sim is verified (built on Apple silicon, run on a macos-15-intel simulator by the new ios-intel-run job). The clang *-windows-msvc row takes { path } for a VS tree or a /winsysroot layout.
• Reports: Using sysroot glibc 2.31 ← … [custom · mcpp.toml:12], mcpp why sysroot, base / execution in resolution.json, refusal code target-base, --managed-only.

The execution host. [target.<triple>].run_under (also in config.toml, overridden by MCPP_TARGET_<TRIPLE>_RUN_UNDER) composes <run_under…> <runner…> <artifact> <args>. Runners receive MCPP_RUN_TRIPLE/KIND/MODE/ID/CLOSURE and MCPP_SYSROOT; the exit status is the program's. An Apple artifact on a non-macOS host without run_under is refused before anything is spawned.

#832, mcpp side. release and dist define NDEBUG and trim the paths an artifact carries (trim_paths: each package to <name>@<version>/…, store/home/build/toolchain/sysroot to /mcpp/…; source-map.gdbinit/.lldbinit for a debugger); both are profile keys and say so once after the upgrade. A pasted command quotes its path. The release workflow checks mcpp's own binary for the build machine's paths (check_release_paths.sh).

Specs: SPEC-006 v0.10 (§3.9–§3.11, appendix A), SPEC-002 v1.2 (§6), SPEC-004 v1.17. Docs 04, 09, 10, 20, 22, 25, 50 (en + zh). Design: .agents/docs/2026-10-23-target-base-and-execution-host-design.md (rev 5 records where the implementation departs from it, and why).

Compatibility

The ecosystem's default build is unchanged (same fingerprint and build.ninja measured on Linux). release/dist rebuild once for ndebug/trim_paths; dev does not. Every existing sysroot spelling keeps its meaning.

Test plan

• Unit: 151 passed locally (new test_target_base: grammar, rows, measurement tables, identity, Apple locator via SDKROOT, link-model rendering)
• e2e (local, Linux): 925 stated sysroot (S1–S4, S6, S7), 926 glibc@system gcc+llvm, 927 runner contract, 928 Apple SDK on Linux, 929 trim_paths/ndebug, 930 toolchain phase base; regression set over sysroot/runner/pack/profile tests
• CI: Linux, macOS (incl. ios-intel-run), Windows; 931/932 need a fresh sandbox (932 after xim-pkgindex#1032 is published)