Move pnpm's version banding and lockstep off the .meta-updater hacks and onto
the integrated versioning engine, then restore the PR-time drift guard the move
removed.
- versioning.epics bands every pnpm11/ library major to the pnpm CLI's major;
versioning.fixed keeps [pnpm, @pnpm/exe, ...artifacts] in exact-version
lockstep. Both live in pnpm-workspace.yaml. Removes the
smallestAllowedLibVersion rewrite, the @pnpm/exe/artifact version forcing,
the EXPERIMENTAL_PKGS set, and the semver dependency from .meta-updater.
- Adds `pnpm change check` (both stacks): a static counterpart to
`pnpm change status` that validates committed versions against the epic bands
and fixed-group lockstep, failing with every violation listed. Backed by
checkVersioningInvariants / check_versioning_invariants in the versioning
engine. Wired into CI via the check:versioning lint step so drift is caught at
PR time, the way meta-updater --test used to catch it.