Capability passed to cleanup callbacks that may need to initiate disposal of
their owner. The request is deliberately non-blocking: cleanup must finish
before the owner's external disposal barrier can settle.
Start disposal of the callback's owner without waiting on the cleanup
callback that made the request. External dispose() callers still await
the complete teardown and receive its failures.
Capability passed to cleanup callbacks that may need to initiate disposal of their owner. The request is deliberately non-blocking: cleanup must finish before the owner's external disposal barrier can settle.