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/friendly/strings/en-US.yaml

@@ -253,6 +253,10 @@ help:
         mode simple|advanced       — switch input mode
     messages: |-
         messages [short|verbose]   — show or switch error-message verbosity (verbose is the default)
+    undo: |-
+        undo                       — undo the last change (with confirmation)
+    redo: |-
+        redo                       — redo the last undone change (with confirmation)
   ddl:
     create: |-
         create table <T> with pk [<col>(<type>), ...]    — create a table
@@ -484,6 +488,8 @@ parse:
     import: "import <zip-path> [as <target>]"
     mode: "mode simple  |  mode advanced"
     messages: "messages  |  messages short  |  messages verbose"
+    undo: "undo"
+    redo: "redo"
 
 # ---- Pre-submit diagnostics (ADR-0027) -------------------------------
 # Surfaced by the validity indicator and the hint panel before
@@ -665,6 +671,25 @@ modal:
   load_picker_path_prompt: "Path to project directory:"
   rebuild_confirm_title: "Rebuild project"
   rebuild_confirm_prompt: "Continue?"
+  # Undo / redo confirmation (ADR-0006 Amendment 1).
+  undo_confirm_title: "Undo last change"
+  redo_confirm_title: "Redo last undone change"
+  undo_confirm_command: "This will undo:"
+  redo_confirm_command: "This will re-apply:"
+  undo_confirm_when: "snapshot taken {timestamp}"
+  undo_confirm_prompt: "Restore that earlier state?"
+  undo_cancelled: "undo cancelled"
+  redo_cancelled: "redo cancelled"
+
+# ---- Undo / redo command surfaces (ADR-0006 Amendment 1) -------------
+undo:
+  disabled: "undo is turned off for this session (started with --no-undo)"
+  nothing_to_undo: "nothing to undo"
+  nothing_to_redo: "nothing to redo"
+  undone_ok: "undone: {command}"
+  redone_ok: "re-applied: {command}"
+  undo_failed: "could not undo: {error}"
+  redo_failed: "could not redo: {error}"
 
 # ---- Save / save-as command surfaces ---------------------------------
 save: