parentScopeCancellationStopsBackgroundJobs

Cancelling the scope create was handed stops the coordinator, with no call to AutoCloseable.close.

ScopedCloseable.ownJob is documented as a structural child of the parent scope's Job, so cancellation propagates down and a caller who simply drops its scope leaks nothing. The failure this catches is a coordinator that owns a parentless job and relies on its own close() to stop it: every other property in this file passes for such a type, because they all reach the coordinator through close(). MuxServerLoom shipped in exactly that shape (#1366), and every module that had noticed the gap was hand-rolling this test locally — :kuilt-quilter against a Quilter, :kuilt-core against a MuxServerLoom — which is why it belongs here rather than in a binding.