Add the grouping $state beside the #780 layer-filter state, render the GroupingControl stacked above the filter trigger (disabled, but kept in place, when no loose letters remain), make the meta-line grouping label track the active mode, and thread groupingMode into TimelineView — filter-then-group, no refetch. Refs #827 Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>