Fix the cancellation leak without muting the race test.

Keep the durable job and delegate semantics intact. Verify the actual stream-close boundary before changing production behavior.

I’ve isolated the ownership transition. The pipe reader survives one cancellation path because completion reaches the reducer before close propagates to the process group.

Delegate reviewer is active. Background race suite has emitted progress for 612 / 842 packages. Context pressure is approaching the next compaction threshold.