A `required` entry without a matching `properties` key made the schema
unsatisfiable: additionalProperties: false (the only supported object
mode) rejects any key not listed in `properties`, so the required key
can never be present. Since no seen bit is emitted for such keys, the
generated parser silently accepted every document instead of rejecting
them as the README contract requires ("missing required property").
Follow the pattern from issue #55 and raise GenError at generation time
when set(required) is not a subset of properties.keys(), for root,
nested, and $defs object schemas alike. Update the README's
not-supported list accordingly.
Closes#63
weaselbot