Replace the flatMap intermediate array plus two filter passes with one
walk over the year bands and the undated bucket. Same counts, no
throwaway allocation.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
A pure timelineMeta() returns the year range (first/last band, null when
there are no bands) and the letter/event totals across all year bands plus
the undated bucket — the single place these counts are computed.
Refs #833
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>