Pure module powering the #827 Datum·Ereignis·Thema toggle: buildEventLookup
(curated events that survived the #780 layer filter), hasLooseLetters (the
control's enabled state), and bucketLetters (cluster loose letters by
linkedEventId or primary root tag, with a "Weitere Briefe"/"Ohne Thema"
fallback). Filter-then-group, no refetch.
Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
