Singular fiber primes

The complete, sorted list of primes $p$ for which the fiber of the realization-space scheme over $\mathbb F_p$ is singular, stored as a string such as "[]" or "[3, 13]". Only fibers that actually exist count, since an empty fiber is smooth. So "[]" does not imply smoothness over ℤ: a nonempty scheme living in a single characteristic is not flat, even if its only fiber is smooth.

Complete for rank 3 on at most 11 elements. The proof route for each record is in rs_singular_fiber_primes_method. This column replaces the legacy singular_primes.

Full details: docs/realization_scheme_singularity_census.md (Section 18.2).