usable() and is_catalog() checked the object number only, so a later stream
with /Root 1 1 R outranked the trailer naming 1 0 R; get_offset then found no
such object and Document Properties lost the catalog. cos resolves by number
alone, so its rank needs no change.
Test: inspect::tests::rebuilt_file_prefers_a_trailer_whose_root_is_the_catalog