whatWasDeletedBeforeARestartIsStillAbsentAfterIt

A DurableStore.delete that happened before the restart must still have happened after it.

The failure this catches is the mirror of the one above and reaches further: a backend whose delete removes the entry from a live map but never from the medium comes back with a value the caller deleted. Every same-handle delete property in this file is green against it, because the live map is what they read.

A sibling key is written and left alone so the assertion distinguishes "the delete was durable" from "the restart lost everything" — without it, a backend whose reopen returns an empty store passes this and its own bug is what makes it pass.

On RestartFixture.KeepsNothing the sibling is expected absent instead, and the deleted key's assertion carries no information there — everything is gone either way. That is stated rather than dressed up: on that arm this property discriminates on the sibling alone.