Undo, Redo, and Back: Stacks and Queues in the UI

LIFO and FIFO made concrete in the interfaces you build every day: undo/redo as two mirrored stacks, the classic bug where a fresh edit must clear the redo stack, the browser's Back button as a history stack, a render/task queue as FIFO, and a deque-backed "recent items" list. Every code trace is verified by running it in Node.