deleteMakesTheKeyAbsentAndWritableAgain

Delete makes the key absent — genuinely absent, the same state readOfANeverWrittenKeyIsNull describes — and leaves it usable again.

The re-write half catches the backend whose delete is a tombstone the write path then honours: a key that can be deleted but not resurrected is a slow leak of the key space, and the caller has no listing surface to notice it with.