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/10-spellgrid/reference/rust/src/harness.rs

@@ -317,6 +317,12 @@ impl Baba {
         kinds
     }
 
+    /// The kinds on tile (x, y), as `kinds_at`, but oldest object first.
+    pub fn stack_at(&self, x: i32, y: i32) -> Vec<String> {
+        let board = self.level.board();
+        board.at(x, y).map(|o| o.kind.to_string()).collect()
+    }
+
     // ---- Module 9 ----
 
     /// Play one whole turn: movement, parse, conversion, parse, hazards, win
@@ -334,6 +340,12 @@ impl Baba {
     pub fn lost(&self) -> bool {
         self.level.lost()
     }
+
+    /// Put the level back as it was before the last `step` or `move_dir`. False, and
+    /// no change, if there is none; repeated calls walk further back.
+    pub fn undo(&mut self) -> bool {
+        self.level.undo()
+    }
 }
 
 fn normalised(rules: &[puzzle::Rule]) -> Vec<String> {
@@ -402,6 +414,10 @@ impl SpellGrid {
         kinds.sort();
         kinds
     }
+    pub fn stack_at(&self, x: i32, y: i32) -> Vec<String> {
+        let board = self.grid.level().board();
+        board.at(x, y).map(|o| o.kind.to_string()).collect()
+    }
     pub fn step(&mut self, dir: Dir) {
         self.grid.step(dir);
     }
@@ -411,6 +427,11 @@ impl SpellGrid {
     pub fn lost(&self) -> bool {
         self.grid.level().lost()
     }
+    /// Not supported: Part I has state (health, hands, the log) that the level's
+    /// snapshot doesn't cover. Panics.
+    pub fn undo(&mut self) -> bool {
+        panic!("undo is not supported in SpellGrid");
+    }
 
     // ---- Module 10 ----