Thread groupingMode through TimelineView → YearBand. TimelineView resolves the
event lookup once over the filtered view (so Ereignis clusters never reference a
filtered-out event). In non-Datum modes YearBand keeps its event pills/world-bands
identical (REQ-001) and replaces the loose letters with per-year LetterBuckets
(REQ-002/003/004); Datum keeps the original card/strip path. The undated bucket is
unchanged in every mode.
Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>