Fix the cancellation leak without muting the race test.

Keep durable jobs and delegate delivery intact. Verify the stream-close boundary before changing production behavior.

The process group is not the failing boundary. A completion event arrives while the reader goroutine still owns the pipe, leaving one path open after cancellation.

Background race suite is active. Reviewer delegate has not reported. Context pressure is near the next fold threshold.

I’m testing the close-order change against the isolated reproducer before starting the full package suite.