The items queued behind one flush — the core of a group commit.

add/3 queues an item and, when the batch was empty, sends flush_msg to the current process once. That message lands behind every request already in the mailbox, so everything that arrives before it joins the same batch; take/1 hands the items back in arrival order and empties the batch. The owner decides what one flush does — one durable sync, one manifest write — and answers every item after it.