Merging pages with conflicting AcroForm /DR must inline the resources
each field's appearance relies on, since only one /DR survives in the
output. That fixup:
- resolved the appearance /Resources ref against the source document's
xref instead of the merged one, so the lookup failed and merging threw
(null.has(...));
- skipped checkbox/radio widgets, whose /AP /N is a sub-dictionary of
appearance states rather than a single stream;
- threw on a non-dictionary /Resources (e.g. a stray name) instead of
falling back to the default resources.