The install root is not only the artifact: `ludic add` caches fetched packages
in <root>/store, keyed by content hash. install_staged moved the whole root
aside and replaced it, so re-running the installer — which is exactly what
`ludic upgrade` does — deleted the cache and forced every project to refetch.
The store is moved into the staged tree before the swap. Nothing else in the
root is preserved, because everything else is the toolchain and should be
replaced.
Verified by staging an install, planting a store entry, upgrading, and reading
the entry back.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
