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/kill-ring.ts

@@ -0,0 +1,46 @@
+/**
+ * Ring buffer for Emacs-style kill/yank operations.
+ *
+ * Tracks killed (deleted) text entries. Consecutive kills can accumulate
+ * into a single entry. Supports yank (paste most recent) and yank-pop
+ * (cycle through older entries).
+ */
+export class KillRing {
+	private ring: string[] = [];
+
+	/**
+	 * Add text to the kill ring.
+	 *
+	 * @param text - The killed text to add
+	 * @param opts - Push options
+	 * @param opts.prepend - If accumulating, prepend (backward deletion) or append (forward deletion)
+	 * @param opts.accumulate - Merge with the most recent entry instead of creating a new one
+	 */
+	push(text: string, opts: { prepend: boolean; accumulate?: boolean }): void {
+		if (!text) return;
+
+		if (opts.accumulate && this.ring.length > 0) {
+			const last = this.ring.pop()!;
+			this.ring.push(opts.prepend ? text + last : last + text);
+		} else {
+			this.ring.push(text);
+		}
+	}
+
+	/** Get most recent entry without modifying the ring. */
+	peek(): string | undefined {
+		return this.ring.length > 0 ? this.ring[this.ring.length - 1] : undefined;
+	}
+
+	/** Move last entry to front (for yank-pop cycling). */
+	rotate(): void {
+		if (this.ring.length > 1) {
+			const last = this.ring.pop()!;
+			this.ring.unshift(last);
+		}
+	}
+
+	get length(): number {
+		return this.ring.length;
+	}
+}