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/rust/test_08.rs

@@ -161,3 +161,22 @@ fn test_08_stacked_you_moves_together() {
     assert_eq!(b.pos_of(first), Some((2, 2)));
     assert_eq!(b.pos_of(second), Some((2, 2)));
 }
+
+#[test]
+fn test_08_stack_is_age_ordered() {
+    let mut b = new_baba(empty_level(3, 3));
+    write(&mut b, &[("BABA", 0, 0), ("IS", 1, 0), ("YOU", 2, 0)]);
+    let baba = b.place("baba", 0, 2);
+    b.place("rock", 1, 2);
+    b.place("key", 1, 2);
+
+    assert_eq!(b.stack_at(1, 2), vec!["rock", "key"]);
+
+    b.move_dir(Dir::Right);
+
+    assert_eq!(b.pos_of(baba), Some((1, 2)));
+    assert_eq!(b.kinds_at(1, 2), vec!["baba", "key", "rock"]);
+    // Oldest first: Baba arrived last, but it was placed first.
+    assert_eq!(b.stack_at(1, 2), vec!["baba", "rock", "key"]);
+    assert_eq!(b.stack_at(0, 2), Vec::<String>::new());
+}