Smooth in characteristic 0

Records whether the characteristic-zero realization space is smooth. It was written together with the legacy singular_primes column (scripts/rank_n_smoothness_and_singular_primes.jl); for rank 3 it was imported from an earlier computation. The repository docs do not describe the method further.

For rank 3 on at most 11 elements, the audited integral census (rs_is_smooth_over_ZZ, rs_singular_fiber_primes) is the authoritative source.