feat: undo in Module 9 and stackAt from Module 8; Lesson 8 quote matches the reference

Baba.undo() restores the level before the last step or move (board snapshot:
objects in age order with ids, the id counter, and won; rules recompiled).
place clears the history. SpellGrid's undo throws/panics 'not supported'.
stackAt / stack_at lists a tile's kinds oldest first, on Baba and SpellGrid.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

course/08-push-chains/tests/ts/test_08.test.ts

@@ -163,3 +163,25 @@ test("test_08_stacked_you_moves_together", () => {
   expect(b.posOf(first)).toEqual([2, 2]);
   expect(b.posOf(second)).toEqual([2, 2]);
 });
+
+test("test_08_stack_is_age_ordered", () => {
+  const b = newBaba(emptyLevel(3, 3));
+  write(b, [
+    ["BABA", 0, 0],
+    ["IS", 1, 0],
+    ["YOU", 2, 0],
+  ]);
+  const baba = b.place("baba", 0, 2);
+  b.place("rock", 1, 2);
+  b.place("key", 1, 2);
+
+  expect(b.stackAt(1, 2)).toEqual(["rock", "key"]);
+
+  b.move("right");
+
+  expect(b.posOf(baba)).toEqual([1, 2]);
+  expect(b.kindsAt(1, 2)).toEqual(["baba", "key", "rock"]);
+  // Oldest first: Baba arrived last, but it was placed first.
+  expect(b.stackAt(1, 2)).toEqual(["baba", "rock", "key"]);
+  expect(b.stackAt(0, 2)).toEqual([]);
+});