TimelineGroup

One rendered band of a timeline - a calendar bucket plus the items that fall into it, already sorted.

The group carries no label: formatting a date is a locale concern that belongs to the *cngxTimelineDateHeader slot (or the config's groupLabel fallback), not to the presenter. key is stable and collision-free across the built-in groupers, so it doubles as the @for track expression and as the aria-labelledby id seed.