snapshotSavedWithoutDiscard_leavesSuffixReadable

RaftStorage.saveSnapshot must be durable before RaftStorage.discardLogPrefix runs, so a crash between the two legally leaves the snapshot plus the un-discarded prefix (#1221). An adapter is not required to have discarded anything at this point, and this suite does not assert that it has.

What it does assert is the read the engine performs regardless of which side of that window a node crashed on: entries(snapshotIndex + 1) returns exactly the suffix above the baseline. The unfiltered log is checked only for the property that holds either way — still contiguous, still ascending, still ending at the tail — so an adapter that keeps the prefix passes and one that mangles the log on saveSnapshot does not.