[codex] Use official mcpp-index packages

[Sunrisepeak]

Summary

Migrates the mcpp build manifest away from the project-local xlings mcpp index:

• removes the checked-in mcpp/ package index and generated config headers
• replaces xlings.libarchive with official-index compat.libarchive
• uses [dependencies.compat] and [dependencies.mcpplibs] namespace tables instead of dotted dependency keys
• drops root-level third-party C library configure macros from mcpp.toml
• keeps only xlings-owned build flags plus LIBARCHIVE_STATIC for C++ units that include libarchive headers
• updates mcpp.lock
• updates CI/release bootstrap to xim:mcpp@0.0.35 via xim-pkgindex commit cf3d0fa64e8be120c3c703c8702f294f271026ad
• clears stale default mcpplibs index checkouts after Actions cache restore so CI sees the newly landed official compat packages

Upstream status

The upstream pieces are landed:

• feat(build): apply package-owned compile flags mcpp-community/mcpp#88 merged
• mcpp 0.0.35 released with package-owned build flags, generated files, quiet xlings index update output, and fixed namespaced local path lookup
• [codex] Add xlings compat compression packages mcpplibs/mcpp-index#17 merged with compat.libarchive and compression dependencies
• feat(mcpp): bump to 0.0.35 xim-pkgindex#259 merged with xim:mcpp@0.0.35

Validation

• local mcpp build with installed mcpp 0.0.35 passed
• earlier glibc/musl validations passed against the same mcpp/mcpp-index changes; see .agents/docs/2026-05-30-official-mcpp-index-migration-plan.md
• current CI is re-running on commit 9fb8695 with default mcpp-index cache refresh enabled

Follow-up

If refreshed CI passes, mark ready, squash merge, then run the xlings release/update pipeline.