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/reference/ts/README.md

@@ -12,6 +12,6 @@ The course's own engine as of the end of Module 8. Part I's card engine is uncha
 - The rest of `src/engine/`: Part I, unchanged.
 - `src/content/movement.ts` (new) and `src/content/index.ts`: the movement responses, in the order the engine runs them: `OPEN`/`SHUT`, `PUSH`, `STOP`. What a property does when something moves is content, not engine.
 - `src/content/words.json`: Module 8 adds `OPEN` and `SHUT`.
-- `src/harness.ts`: the adapter the course tests call. Module 8 adds `move`, `posOf` and `kindsAt` on `Baba`.
+- `src/harness.ts`: the adapter the course tests call. Module 8 adds `move`, `posOf`, `kindsAt` and `stackAt` on `Baba`.
 
 Run it with the module's tests: `node tools/verify-references.mjs 08` from the course repository.

course/08-push-chains/reference/ts/src/harness.ts

@@ -155,6 +155,7 @@ export interface Baba {
   move(dir: puzzle.Dir): void; // every YOU object one tile, with what it pushes; then reparse. Movement only.
   posOf(id: Id): [number, number] | null; // null once destroyed
   kindsAt(x: number, y: number): string[]; // kinds as placed ("baba", "text:ROCK"), sorted
+  stackAt(x: number, y: number): string[]; // the same tile, oldest object first
 }
 
 /** Distinct rules as normalised strings, sorted. */
@@ -178,5 +179,9 @@ export function newBaba(level: puzzle.LevelDef): Baba {
         .at(x, y)
         .map((o) => puzzle.Kind.format(o.kind))
         .sort(),
+    stackAt: (x, y) =>
+      lvl.board
+        .at(x, y)
+        .map((o) => puzzle.Kind.format(o.kind)),
   };
 }