Singular fiber characteristics

The complete set of primes $p$ for which the fiber of the realization-space scheme over $\mathbb F_p$ is singular. Fibers that are empty count as smooth. This is the rank-3, 12-element counterpart of singular fiber primes. It needs a richer encoding because 157 of these schemes are singular in characteristic 0 and at all but finitely many primes.

The value is either a finite set of primes ("none" when the set is empty) or "all primes except" a finite list. The proof route for each record is in rs_singular_fiber_characteristics_method.

Full details: docs/r3_n12_census.md.