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.

Cargo.toml

@@ -18,7 +18,7 @@ futures-util = "0.3.32"
 gethostname = "1.1.0"
 rand = "0.10.1"
 ratatui = "0.30.0"
-rusqlite = { version = "0.39.0", features = ["bundled", "column_metadata"] }
+rusqlite = { version = "0.39.0", features = ["backup", "bundled", "column_metadata"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_yml = "0.0.12"
 sysinfo = { version = "0.39.0", default-features = false, features = ["system"] }