exitDeltaLateTaskId

Synthetic task id for exit-mode writes of a superstep after the anchor's own.

Sorts after every real task id, in step order, so replay keeps them after the anchor's own superstep whether a saver orders writes by task path (writesSortKey) or by task id. Mirrors Python's exit_delta_late_task_id exactly, including the two hexadecimal step groups (step is split across them).