Add the kill ring and undo features to the Input component (#1373)

* 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()

packages/tui/src/undo-stack.ts

@@ -0,0 +1,28 @@
+/**
+ * Generic undo stack with clone-on-push semantics.
+ *
+ * Stores deep clones of state snapshots. Popped snapshots are returned
+ * directly (no re-cloning) since they are already detached.
+ */
+export class UndoStack<S> {
+	private stack: S[] = [];
+
+	/** Push a deep clone of the given state onto the stack. */
+	push(state: S): void {
+		this.stack.push(structuredClone(state));
+	}
+
+	/** Pop and return the most recent snapshot, or undefined if empty. */
+	pop(): S | undefined {
+		return this.stack.pop();
+	}
+
+	/** Remove all snapshots. */
+	clear(): void {
+		this.stack.length = 0;
+	}
+
+	get length(): number {
+		return this.stack.length;
+	}
+}