A smart collection stores its selector as JSON in collections.selector_json,
and cache rules carry one across a sync, so the predicate language has to
round-trip. Derive-only: no serde machinery leaks into the rest of core/.
FlagState gains Default = Unflagged, which is what an image is before anyone
has looked at it. Any other default would assert a judgement the
photographer never made.
Assisted-by: LLM
dtourolle
b6a5c0f090