Pure helpers for the /zeitstrahl layer filter: isDefaultState and
hiddenLayerCount drive the "Filter (N active)" trigger, and filterTimeline
derives a client-side view that hides personal/historical/letter layers and
drops year bands left empty. Letters ride the Letters layer, HISTORICAL events
the Historical layer, and curated PERSONAL plus derived life-events the
Personal layer.
Refs #780
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>