Relocate the 10 pure helpers (monthBoundaryFrom/To, buildMonthSequence,
fillDensityGaps, clipBucketsToRange, aggregateToYears, selectionBoundaryFrom/To,
tickIndicesFor, formatTickLabel) and their unit tests out of document/timeline.ts
into a shared module so lib/timeline/ can consume them without importing
lib/document/. The /api/documents/density glue (buildDensityUrl, fetchDensity,
DensityState, DensityFilters) stays in document/timeline.ts. Re-point the three
density components and the density-filter spec at the shared module.
Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
