Characteristic dimensions method

How rs_characteristic_dimensions was computed:

• empty_characteristic_set: the characteristic set is empty, so the list is [].
• finite_exact_fiber_saturation: finite characteristic set. Every listed prime's fiber was computed exactly (reduce, then saturate by the inequations).
• cofinite_strong_groebner_candidates_exact: cofinite set. A strong Gröbner basis over $\mathbb Z$ from a historical scan gave a finite superset of the primes where good reduction can fail. Each realized candidate was recomputed exactly, and all other primes have the generic dimension.
• cofinite_smooth_equidimensional: cofinite set, for records without a usable historical candidate list. The tag is not spelled out in the docs; its name points to smoothness and equidimensionality certificates.
• simple_core: copied from the simple core.

Full details: docs/characteristic_dimensions.md.