Group API

Core group configuration, exclusive hierarchy, callbacks, and resolver API.

GroupConfig extends NodeConfig with width, height, canContain, and groupCallbacks.

Member	Meaning
`members`	Read-only direct member set
`descendants`	Read-only recursive member set
`parentGroup`	Settled direct parent or `null`
`refreshMembership(fireDelta)`	Reconcile engine membership after an external geometry change

GroupMembershipEvent contains added, removed, and the final direct members. setGroupMembershipResolver(engine, resolver) overrides the default innermost candidate and returns a cleanup function.