sync_venv took four mutually exclusive plugin kwargs (plugin_dirs,
extra_plugin_dirs, selection, staged_plugin) and venv_is_current two, with
runtime ValueErrors guarding the combinations. Replace them with a single
`plugins=` argument typed as one of pm.plugin_inputs.Members, Candidates,
Selection or StagedUpdate, so a conflicting request cannot be expressed.
The module also owns the worker wire encoding that client.py and worker.py
each duplicated.
pm.install.sync_venv is split into cohesive helpers (feature policy,
install lock, publication snapshot, target selection, commit) with the
same ordering, receipts and recovery; its CC drops from 54 to 16.
All in-tree callers and tests move to the new argument.
