Coverage is indexed by shape and answers 'does a field name this item'. Two
saves carrying content nobody had modelled scored 100% anyway, because the
novelty landed in fields that were already typed. This asks what the corpus has
ever actually put in each field.
Over 22 saves: 724 fields observed, 490 vary, 234 constant. A third of the
format we call 99.99% typed has been seen holding exactly one value.
