Splits fixture mismatches into classes that name the responsible
subsystem: plain-vs-plain cells are direct base-terrain defects and come
with sample coordinates, single-side feature states count as flips per
family, and disk-shaped outputs land in an ambiguous bucket so they
cannot pollute the plain signal.
First run on seed 12345: 966 base cells (0.246%), dominated by vanilla-
carved cave segments missing from our carver output and deepslate-to-
water aquifer edges.
