isDense(count) thresholds dense year bands at >12 letters (REQ-012);
monthHistogram(letters, year) buckets a band's letters into exactly 12 month
buckets via the shared fillDensityGaps, counting each letter on its eventDate
anchor month and ignoring undated entries (REQ-027). Imports shared only.
Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
