contextualToolbarGroupingProvider

Optional host-supplied grouping override for contextual toolbars, or null (the default) to use the SDK preset for the current capacity. Read by the contextual toolbar composable on every (re-)measure; snapshot-backed so a provider registered or cleared while a bar is shown regroups it immediately. Like action lambdas, the provider is never persisted — hosts re-register it on restore (ADR-Toolbar-4).