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/rust/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.rs` (new) and `src/content/mod.rs`: 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.rs`: the adapter the course tests call. Module 8 adds `Baba::move_dir`, `Baba::pos_of`, `Baba::kinds_at`, and re-exports `Dir`.
+- `src/harness.rs`: the adapter the course tests call. Module 8 adds `Baba::move_dir`, `Baba::pos_of`, `Baba::kinds_at`, `Baba::stack_at`, and re-exports `Dir`.
 
 Run it with the module's tests: `node tools/verify-references.mjs 08` from the course repository.