ci-linux e2e 29/31 fail deterministically: gcc@16.1.0 first-run install — glibc 2.39 xpkg missing linux/limits.h #120

29_toolchain_partial_versions.sh and 31_transitive_deps.sh fail on main (run 26988545086, plus a rerun) and on unrelated PRs (e.g. #119 run 27007239457) with the same signature — deterministic, not a flake:

/…/.mcpp/registry/data/xpkgs/xim-x-glibc/2.39/include/bits/local_lim.h:38:10:
fatal error: linux/limits.h: No such file or directory

during the std-module precompile right after the tests' first-run install of gcc@16.1.0 (glibc, native ABI).

Observations:

• The xim-x-glibc/2.39 package ships glibc headers but no Linux kernel headers (linux/limits.h et al.), which bits/local_lim.h includes unconditionally.
• The injected runtime path pairs xim-x-gcc/16.1.0 with xim-x-gcc-runtime/15.1.0 — the 16.1.0 toolchain package set looks recently published and incompletely paired.
• Earlier main runs (gcc 15.x default era) passed these tests; the breakage tracks the registry-side default moving to 16.1.0, not any mcpp code change (e2e 65/67 otherwise green on the same runs).

Suggested fixes (registry-side): add kernel headers to the glibc xpkg (or a separate linux-headers xpkg included by the toolchain meta), and pair gcc 16.1.0 with its own runtime package.