Export wrote each keyword's `a|b` path into `dc:subject` and nothing
into `lr:hierarchicalSubject`. It now writes what
`Catalog::export_keywords` gives: names flat in `dc:subject` (with the
keywords containing them and synonyms, as their options say), full
paths in `lr:hierarchicalSubject`, and nothing of a keyword left out
of export. `export_metadata` takes the catalog for the keyword list.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>