docs: ADR-0006 §8 step 8 — mark undo/snapshot implemented

- requirements.md U1/U2: [ ] -> [x] with implementation notes
- CLAUDE.md Safety bullet: implemented; drop U-series from the
  deliberately-deferred list
- ADR-0006 Amendment 1: "not yet implemented" -> implemented, plus an
  Implementation note (index.yaml, source-gated snapshots, non-fatal
  snapshot-failure policy, batch primitive, testing + accepted gaps)
- ADR README index: undo/snapshot half marked implemented

docs/requirements.md

@@ -365,7 +365,7 @@ handoff-14 cleanup; 449 after B2/C2.)
 
 ## Undo and replay (per ADR-0006)
 
-- [ ] **U1** Auto-snapshot before **every** data/schema mutation
+- [x] **U1** Auto-snapshot before **every** data/schema mutation
   (DSL + SQL) into a persisted ring buffer (size N=50, tunable),
   per ADR-0006 Amendment 1 (single-step undo, superseding the
   original destructive-only model). Snapshot is a hybrid
@@ -374,14 +374,19 @@ handoff-14 cleanup; 449 after B2/C2.)
   finalised after the db commit (preserves ADR-0015 §6). A batch
   command (`replay` / future batch ops) records one boundary
   snapshot; `import` takes none. A `--no-undo` CLI flag disables
-  snapshotting. *(Designed + amended + planned in
-  `docs/plans/20260524-adr-0006-undo-snapshots.md`; not yet
-  implemented.)*
-- [ ] **U2** `undo` restores the most recent snapshot (database +
+  snapshotting. *(Implemented 2026-05-24, plan
+  `docs/plans/20260524-adr-0006-undo-snapshots.md`: `src/undo.rs`
+  ring + worker dispatch hook in `src/db.rs`; snapshots gated on a
+  user command `source` so internal ops like open-time rebuild are
+  not recorded; snapshot-bookkeeping failures are non-fatal, restore
+  failures surface.)*
+- [x] **U2** `undo` restores the most recent snapshot (database +
   text, directly); `redo` re-applies (redo stack discarded on new
   work); both prompt for confirmation naming the command being
-  undone / re-applied (`Y` confirms). *(Designed + amended; not yet
-  implemented — see U1's plan.)*
+  undone / re-applied (`Y` confirms). *(Implemented 2026-05-24:
+  `undo` / `redo` app commands, `Modal::UndoConfirm`, runtime
+  prepare→confirm→restore→refresh; `--no-undo` reports undo is off,
+  empty stacks report "nothing to undo/redo".)*
 - [x] **U3** `history.log` records every submitted command in
   append-only form, tagged with its outcome (Iteration 2;
   broadened by ADR-0034). Format: `<ISO-8601 Z>|<status>|<source>`