Add rustdoc overviews for a new litellm-router-core crate and for
litellm-model-catalog, a module docstring on litellm/router.py, and a
shrink-only allowlist of the Router members and router_utils modules used
outside the router, checked in CI by router_surface_check.py.
Move the router-free helpers out of litellm/router_utils: batch_utils to
litellm_core_utils, and the auto-router naming contract, tuning baseline
and baseline request next to the auto-router plugin. No behaviour change.
