Groups accumulated during the pipeline can reference items
that a later processor suppressed. The engine now strips
stale IDs and drops empty groups before returning.
Co-authored-by: Ona <no-reply@ona.com>
Add FeedPostProcessor type and FeedEnhancement interface.
Post-processors run after item collection on all update
paths (refresh, reactive context, reactive items).
Pipeline is chained — each processor sees items as modified
by the previous one. Enhancement merging handles additional
items, suppression, and grouped items. Throwing processors
are caught and recorded in FeedResult.errors.
Co-authored-by: Ona <no-reply@ona.com>
