[gaborbernat]

Pin the simplint 0.12.1 hook for contributor and CI checks, and bring the code in line with the rules it adds.

Module-level lookup tables that nothing changes are now read-only (MappingProxyType or tuples), the prompt matrix crosses its states, actions and payloads with itertools.product, helper classes and functions sit below their first users, and two instance attributes nothing reads are gone. The Ruff unnecessary-regular-expression-compile ignore is gone, since simplint no longer conflicts with it.