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/lib.rs

@@ -24,3 +24,4 @@ pub mod runtime;
 pub mod theme;
 pub mod type_change;
 pub mod ui;
+pub mod undo;