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/09-turn-pipeline/reference/ts/src/engine/puzzle/level.ts

@@ -7,8 +7,12 @@
 //
 // A turn (Module 9) runs the phases of `TURN` in order: movement, parse,
 // conversion, parse, content's hazards, the win check, parse.
+//
+// Undo (Module 9): before each move or turn the level saves the board (every
+// object's id, kind and position, oldest first, and the id counter) and `won`.
+// The rules are not saved: undo restores the board and reparses.
 
-import { Board, Kind, type LevelDef, type ObjectId } from "./grid";
+import { Board, Kind, type BoardState, type LevelDef, type ObjectId } from "./grid";
 import { delta, Planner, type Dir, type Plan, type Response } from "./movement";
 import { isConversion, isIdentity, parse, type Rule } from "./parse";
 import { resolve } from "./resolve";
@@ -31,6 +35,8 @@ export class Level {
   private compiled: Rule[] = [];
   private active: Rule[] = [];
   private hasWon = false;
+  /** What `undo` puts back: one entry per move or turn, the latest last (Module 9). */
+  private history: { board: BoardState; won: boolean }[] = [];
 
   /**
    * A level from its definition, the words content provides, and content's
@@ -50,6 +56,7 @@ export class Level {
 
   place(kind: string, x: number, y: number): ObjectId {
     const id = this.board.place(Kind.parse(kind), x, y);
+    this.history = [];
     this.reparse();
     return id;
   }
@@ -110,12 +117,14 @@ export class Level {
    * only: no conversion, hazard or win.
    */
   moveYou(dir: Dir): void {
+    this.save();
     this.movement(dir);
     this.reparse();
   }
 
   /** Play one turn: the phases of `TURN`, in order. */
   step(dir: Dir): void {
+    this.save();
     for (const phase of TURN) {
       switch (phase) {
         case "movement":
@@ -137,6 +146,23 @@ export class Level {
     }
   }
 
+  /**
+   * Put the level back as it was before the last `moveYou` or `step` (Module 9),
+   * and reparse. False, and no change, if there is none; `place` forgets them all.
+   */
+  undo(): boolean {
+    const last = this.history.pop();
+    if (last === undefined) return false;
+    this.board.restore(last.board);
+    this.hasWon = last.won;
+    this.reparse();
+    return true;
+  }
+
+  private save(): void {
+    this.history.push({ board: this.board.snapshot(), won: this.hasWon });
+  }
+
   /**
    * The `YOU` objects move one at a time, the one furthest ahead in `dir` first
    * (oldest first on a tie); each one's move is planned in full, then committed,