/runda found silent data loss: with the non-fatal snapshot-failure
policy, a committed mutation whose snapshot couldn't be staged left
the redo stack stale (redo-clear was only a side effect of finalize),
so a later redo silently discarded the new work. Same gap in batches.
- SnapshotStore::clear_redo() drops the redo stack + payloads
- snapshot_then / end_batch call it when committed user work has no
staged snapshot; for disk-full it succeeds where a full backup
couldn't (tiny index write + payload deletes)
- unit test + integration regression (forced staging failure)
- ADR-0006 implementation note records the fix + residual edge
1698 passed / 0 failed / 1 ignored; clippy clean.