Fails when a citation of a specification section in spec/, lib/, test/ or specs/
names a heading its document does not have: spec/NN §id, spec/NN-slug.md §id, a
specification document's [NN-slug](NN-slug.md) §id, and every id of a list or range
that continues one. spec/13's D-N headings are ids; a heading in a code fence is not.
mix vis.citations