feat: ADR-0006 §8 steps 1-2 — --no-undo flag + snapshot ring module

Step 1 (Cargo + CLI):
- add the `backup` feature to rusqlite (online backup API)
- `--no-undo` flag (test-first) + help-banner entry

Step 2 (snapshot store, src/undo.rs):
- SnapshotStore: a persisted undo ring + redo stack under
  <project>/.snapshots/ (index.yaml + per-snapshot payload dirs)
- hybrid whole-project snapshot: db via backup API + project.yaml /
  data/*.csv copied as files; restore is text-first, db-last
  (ADR-0015 §6 commit-db-last)
- stage/finalize/discard, undo/redo (each snapshots current to keep
  the inverse possible), N=50 eviction, redo-cleared-on-new-work,
  orphan/staging cleanup, monotonic ids
- 12 Tier-1 tests; adds a crate-visible persistence::utc_iso8601_now

No worker wiring yet (step 3). 1674 passed / 0 failed / 1 ignored; clippy clean.

src/persistence/mod.rs

@@ -32,6 +32,14 @@ mod yaml;
 pub(crate) use csv_io::{decode_cell, parse_csv};
 pub(crate) use yaml::parse_schema;
 
+/// Current UTC time as an ISO-8601 `Z` string — the same shape
+/// `history.log` records (ADR-0015 §5). Exposed crate-wide so the
+/// undo snapshot ring (ADR-0006) timestamps entries identically.
+#[must_use]
+pub(crate) fn utc_iso8601_now() -> String {
+    history::utc_iso8601_now()
+}
+
 /// Owns persistence to a single project on disk. Cheap to
 /// move; the db worker holds one instance for its lifetime.
 #[derive(Debug, Clone)]