refactor: clean up unused code and update stack demo

- Remove unused assignRef helper function
- Remove unused itemPrefixCls and noticeWrapperCls variables
- Make placement prop required in NotificationListProps
- Update stack demo to use geek.less and inline motion config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zombieJ

docs/examples/stack.tsx

@@ -1,40 +1,54 @@
 /* eslint-disable no-console */
 import React from 'react';
-import '../../assets/index.less';
+import type { CSSMotionProps } from '@rc-component/motion';
+import '../../assets/geek.less';
 import { useNotification } from '../../src';
-import motion from './motion';
 
 const Context = React.createContext({ name: 'light' });
 
+const motion: CSSMotionProps = {
+  motionName: 'notification-fade',
+  motionAppear: true,
+  motionEnter: true,
+  motionLeave: true,
+};
+
 const getConfig = () => ({
   description: `${Array(Math.round(Math.random() * 5) + 1)
     .fill(1)
     .map(() => new Date().toISOString())
     .join('\n')}`,
-  duration: null,
+  duration: false as const,
 });
 
 const Demo = () => {
-  const [{ open }, holder] = useNotification({ motion, stack: true, closable: true });
+  const [{ open }, holder] = useNotification({
+    motion,
+    stack: { threshold: 3, offset: 20 },
+    closable: true,
+    prefixCls: 'notification',
+  });
 
   return (
     <Context.Provider value={{ name: 'bamboo' }}>
-      <button
-        type="button"
-        onClick={() => {
-          open(getConfig());
-        }}
-      >
-        Top Right
-      </button>
-      <button
-        type="button"
-        onClick={() => {
-          open({ ...getConfig(), placement: 'bottomRight' });
-        }}
-      >
-        Bottom Right
-      </button>
+      <div style={{ marginBottom: 16, display: 'flex', flexWrap: 'wrap', gap: 8 }}>
+        <button
+          type="button"
+          onClick={() => {
+            open(getConfig());
+          }}
+        >
+          Top Right
+        </button>
+        <button
+          type="button"
+          onClick={() => {
+            open({ ...getConfig(), placement: 'bottomRight' });
+          }}
+        >
+          Bottom Right
+        </button>
+      </div>
       {holder}
     </Context.Provider>
   );

src/NotificationList.tsx

@@ -27,7 +27,7 @@ export interface NotificationListConfig extends Omit<NotificationProps, 'prefixC
 export interface NotificationListProps {
   configList?: NotificationListConfig[];
   prefixCls?: string;
-  placement?: Placement;
+  placement: Placement;
   pauseOnHover?: boolean;
   classNames?: NotificationClassNames;
   styles?: NotificationStyles;
@@ -40,14 +40,6 @@ export interface NotificationListProps {
   onAllRemoved?: (placement: Placement) => void;
 }
 
-function assignRef<T>(ref: React.Ref<T>, value: T | null) {
-  if (typeof ref === 'function') {
-    ref(value);
-  } else if (ref) {
-    (ref as React.MutableRefObject<T | null>).current = value;
-  }
-}
-
 const NotificationList: React.FC<NotificationListProps> = (props) => {
   const {
     configList = [],
@@ -118,8 +110,6 @@ const NotificationList: React.FC<NotificationListProps> = (props) => {
 
   // ========================= Render =========================
   const listPrefixCls = `${prefixCls}-list`;
-  const itemPrefixCls = `${listPrefixCls}-item`;
-  const noticeWrapperCls = `${prefixCls}-notice-wrapper`;
 
   return (
     <div