Block

Ends the run as a finished failure with error, which is what a guardrail's Block is. update commits with the superstep like any other, together with every sibling's result, and the run's closing checkpoint is CheckpointStatus.Failed: the thread is usable afterwards and recover refuses it. The caller receives error itself, not a GraphError.NodeFailed around it. A blocked task records no pending write, so a process that dies before the closing commit runs the task again.