ToolbarSubscription

Handle returned when a producer (e.g. a PdfFragment lifecycle owner) binds itself to a ToolbarCoordinator.

The producer must call close in its teardown callback (onDestroyView for fragments, DisposableEffect cleanup for Compose). After close, the coordinator no longer holds any reference to the producer — this is what makes the leak-free guarantee from loophole P0-3 enforceable.

Pre-Phase 0 scaffolding.