GameSim snapshots the state before each undoable command
(PlacePiece / RemovePiece / MovePiece) into a bounded LinkedList stack
(max 32). Undo() pops the last checkpoint and emits StateRestoredEvent,
reusing the presentation rebuild path already wired for QuickLoad.
Ctrl+Z in Main triggers the engine method; the harness exposes undo()
for tests. QuickLoad clears the stack (fresh timeline). Seven unit tests
cover empty stack, place/remove/move undo, reverse-order multiple undos,
rejected commands not checkpointing, and post-simulation rewind.
