Annotation-local attachments (those not in the catalog `/Names` tree) were
resolved through a dictionary cache that `Catalog.cleanup` clears, so their
content became unreachable once the idle cleanup had run.
Encode the reference of the embedded content in the attachment id and re-fetch
it from the xref on demand instead of caching the dictionary, so the content
stays reachable without anything having to survive cleanup.
It fixes a regression introduced by #21351.
9d9fb06d7f