* feat(tui): extract KillRing and UndoStack, add to Input
Extract kill ring and undo logic from Editor into reusable classes:
- KillRing: ring buffer with accumulation for consecutive kills
- UndoStack<S>: generic stack with clone-on-push semantics
Refactor Editor to use both classes. Add kill ring (kill/yank/
yank-pop), undo with coalescing, and deleteWordForward to Input.
* feat(tui): extract handleBackspace() and handleForwardDelete()
