Keywords existed only as `a|b|c` strings on photos: one couldn't be
created before a photo had it, nor a parent made to organize under.
The catalog now has a keyword list (`Op::SetKeyword`, format version
4): a keyword listed on its own is in the tree with no photos, its
parents too, and carries Lightroom Classic's keyword tag options
(`KeywordInfo`: synonyms, include on export, export containing
keywords, export synonyms, person), exported by default. Undo restores
what was listed, as it was written. Version 3 libraries open and are
rewritten as 4.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Nathaniel LedfordandClaude Opus 5.5
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