Toolbar Preset Entry
One entry of a capacity-keyed contextual-toolbar grouping preset: either a single item reference (childIds null) or a group whose children collapse into a ToolbarItem.Submenu.
The Compose successor to the legacy grouping/presets/MenuItem. Carries the group's stable actionId alongside the legacy id so a synthesized submenu participates in action dispatch and state save/restore like any other item (ADR-Toolbar-4). Custom presets should namespace their group action ids (e.g. "my.app.group.shapes"); pspdf.* is reserved for SDK items.