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.
claude@clouddev1
64eee3ed6d