A write's subject is built from a read taken before its turn in the repository's queue, and `admit` re-reads under the store lock; the two are now compared, and a key that moved refuses the write as `InvalidSwap`. That also closes the delete shortcut: a delete whose first read found an empty key never reaches the gate, so it must not remove a record that landed while it was on its way. Change-Id: Ie719d3b53747e77c2818cf86198d812ac6039c52