snapshotAtZeroBaseline_roundTrips

A snapshot saved at the zero baseline must come back present, with both metadata fields intact.

0 is the lower bound RaftEngine.checkedRestoredSnapshotMeta admits on both halves, and the engine distinguishes "no snapshot" (loadSnapshot() == null) from "a snapshot at index 0" — the latter still seeds state.snapshotConfig, the membership baseline a node that compacted past a config change recovers under. An adapter that encodes absence as a 0 sentinel, or whose metadata columns are nullable-with-default, loses that config and comes back under the wrong cluster configuration.