Ribbon Collapse

Headless group-collapse system — the mechanism behind a classic (expanded) Fluent 2 Ribbon. When a two-row band cannot fit its container it collapses whole groups (highest collapsePriority first — Word: Parágrafo before Fonte) from their expanded form to a single collapsed dropdown button; when every group is collapsed and it still overflows it reports a scrollMode flag (the terminal horizontal-scroll fallback). Ships a framework-agnostic manager (createGroupCollapseManager) plus React bindings: GroupCollapse provider, CollapseGroup (renders both the expanded and collapsed forms and toggles them), and the useGroupMode / useIsScrollMode hooks. ResizeObserver-driven, accurate space accounting (per-group sizes + flex gap + container padding), size caching with an optimistic collapsed estimate, a hide-only settle() safety net, and a referentially-stable subscribe/getSnapshot store built for useSyncExternalStore. SSR-safe (all groups expanded until measured) and headless (no styling of its own). Sibling of the single-line 'overflow' manager.