Mock data lived in lib/ and was re-exported through the slice's
production index.ts, advertising fixtures as part of the public API.
Move it to a testing/ segment and drop it from index.ts; consumers now
import explicitly via the $entities/Font/testing subpath.
Repoints the four mock consumers (FontApplicator story, sizeResolver
and fontCatalogStore specs, comparisonStore test) to the new subpath.
Ilia Mashkov