The declared C runtime is installed before the fixup consumes it; the lookup is exact (#660)

[Sunrisepeak]

Summary

Closes #660.

Cause. The default SubOS declares its glibc by exact version (glibc@2.44.3) and the toolchain fixup patches against xim-x-glibc/<version>, but no step installed that payload. The two loops meant to do it (lifecycle.cppm, first-run in prepare.cppm) passed xim:glibc without a version; Fetcher::resolve_xpkg_path rejects such a target and the rejection was logged at debug level and discarded (mcpp -v shows installing dep: xim:glibc followed by no fetcher line). The payload therefore existed only when xlings installed it as a dependency of a toolchain. A toolchain restored from a CI cache is not reinstalled, so a cache holding another glibc revision left the declared payload absent. The 2.44.x refinement scan in payload_dir_for_version concealed this until two revisions coexisted, which xim-pkgindex#852 (glibc 2.44.3) produced.

Measured with the released 2026.9.15.1 and xlings 2026.9.14.1: a cache-shaped store (gcc payload present, glibc 2.44.2 present, 2.44.3 declared) fails at the first fixup; upgrading xlings alone does not fix it.

Change.

• ensure_declared_runtime installs xim:glibc@<declared> through xlings and re-resolves the binding. It runs once, before the first fixup that consumes a C runtime (post_install_fixup_kind non-empty: gcc and llvm payloads), only on Linux, only for the glibc provider, and only when the binding locates no payload. A home in its normal state starts no process; system, musl and PE toolchains download nothing. A failed install is not fatal at that point; the fixup reports the absence with the coordinate xim:glibc@<version>.
• The lookup is exact: select_glibc_payload_lib and probe's payload_root_for_binding accept only xim-x-glibc/<version>. payload_dir_for_version and the now-unused needs_linux_sysroot_payloads are removed.
• The xlings released beside the mcpp executable (<prefix>/registry/bin/xlings) is an acquisition source ahead of the PATH. An mcpp installed as an xlings package uses ~/.mcpp and previously kept an older PATH xlings with a note.
• Docs: docs/91-toolchain-internals.md (en/zh) states who installs the exact payload. CHANGELOG. Analysis and plan records in .agents/docs/2026-09-17-*.
• Version 2026.9.17.2.

Test plan

• mcpp test: 121 passed (new GlibcPayload.*, DeclaredRuntime.*)
• e2e 737 (new): fresh install, reduce the home to a CI cache shape, rename the declared glibc; offline build fails naming xim:glibc@<v>, online build installs it and runs, toolchain install likewise. Green on this build; red on released 2026.9.17.1 with the toolchain post-install fixup: glibc@2.44 binding cannot resolve after 2.44.3 was published (every clean machine fails) #660 message.
• e2e 687 leg C (new): release layout replaces an older vendored xlings with no override and a reduced PATH. Green here; red on 2026.9.17.1.
• e2e 01, 237, 26 green locally
• check_docs_style.sh, check_docs_structure.sh, check_version_pins.sh
• CI