feat: ADR-0006 §8 steps 4-5 — undo/redo commands + confirm-modal flow

Commands & grammar (step 4):
- AppCommand::Undo/Redo, grammar nodes + REGISTRY entries, catalog
  help/usage + keys; parse tests
- replay skips undo/redo (is_app_lifecycle_entry_word) + completion
  entry-keyword lockstep; replay-skip test extended

Wiring (step 5):
- Action::{PrepareUndo,PrepareRedo,Undo,Redo} + AppEvent::{UndoPrepared,
  UndoUnavailable,UndoSucceeded,UndoFailed}
- App: undo_enabled flag, Modal::UndoConfirm, dispatch + event handling
  + confirm-key handler (Y confirms / N/Esc cancels); "turned off" when
  --no-undo; "nothing to undo/redo" when empty
- ui::render_undo_confirm names the command + snapshot time
- runtime: opens with undo enabled (!--no-undo), threads it through the
  project-switch path, spawn_prepare_undo/spawn_undo (peek->modal,
  restore->refresh tables + schema cache)
- 9 Tier-1 app tests + 3 parse tests

1692 passed / 0 failed / 1 ignored; clippy clean.

src/event.rs

@@ -115,6 +115,33 @@ pub enum AppEvent {
     RebuildFailed {
         error: String,
     },
+    /// Runtime peeked the snapshot `undo`/`redo` would restore and
+    /// is ready for the user to confirm (ADR-0006 Amendment 1). App
+    /// opens the confirmation modal naming `command`. `is_redo`
+    /// selects undo vs redo wording.
+    UndoPrepared {
+        command: String,
+        timestamp: String,
+        is_redo: bool,
+    },
+    /// Nothing to undo / redo (the ring or redo stack was empty).
+    /// App surfaces a friendly note.
+    UndoUnavailable {
+        is_redo: bool,
+    },
+    /// Undo / redo completed. App closes the modal, notes the
+    /// command that was undone/redone; the runtime also refreshes
+    /// the table list + schema cache.
+    UndoSucceeded {
+        command: String,
+        is_redo: bool,
+    },
+    /// Undo / redo failed (a rare restore error). App closes the
+    /// modal and surfaces the error.
+    UndoFailed {
+        error: String,
+        is_redo: bool,
+    },
     /// Runtime has gathered the list of available projects
     /// for the load picker. App opens the picker modal.
     LoadPickerReady {