feat(ui): 新增 Caret / SortableList，箭头画法收成一份

- Caret：6px CSS 折角（open 切朝向，size 可调），Menu 与 Select 改为复用它，
  两处各自的画法删掉，全库只剩一份
- SortableList：可拖拽排序的条目列表（items / activeKey / onSelect / onReorder），
  拖拽位置计算从应用里挪进组件，顺序没变不回调
- 展示页：菜单补「折叠箭头」，新增 #sortable 一节并挂到导航
- COMPONENTS.md 契约 3.48 / 3.49、组件地图、README 清单、变更记录同步

site/sections/MenuSection.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import { Card, Icon, Menu, Text } from '../../src'
+import { Card, Caret, Icon, Menu, Space, Text } from '../../src'
 import { Demo } from '../components/Demo'
 import { PropsTable } from '../components/PropsTable'
 import { Section } from '../components/Section'
@@ -79,6 +79,28 @@ export function MenuSection() {
         </Card>
       </Demo>
 
+      <Demo
+        title="折叠箭头"
+        desc="Caret 是菜单、下拉、可折叠表格行共用的 6px 折角：收起朝下、展开朝上，纯装饰，无障碍状态由外层按钮的 aria-expanded 承担。"
+        code={`<Caret />
+<Caret open />`}
+      >
+        <Space gap="lg">
+          <Space gap="sm" align="center">
+            <Caret />
+            <Text size="sm" tone="muted">
+              收起
+            </Text>
+          </Space>
+          <Space gap="sm" align="center">
+            <Caret open />
+            <Text size="sm" tone="muted">
+              展开
+            </Text>
+          </Space>
+        </Space>
+      </Demo>
+
       <PropsTable
         rows={[
           { name: 'items', type: 'MenuItem[]', required: true, desc: '{ key, label, icon?, children? }' },
@@ -88,6 +110,13 @@ export function MenuSection() {
           { name: 'itemHeight', type: 'number', defaultValue: '36', desc: '一级菜单行高（px）' },
         ]}
       />
+
+      <PropsTable
+        rows={[
+          { name: 'open', type: 'boolean', defaultValue: 'false', desc: '展开状态（决定箭头朝向）' },
+          { name: 'size', type: 'number', defaultValue: '6', desc: '折角边长（px）' },
+        ]}
+      />
     </Section>
   )
 }

site/sections/Sortables.tsx

@@ -0,0 +1,88 @@
+import { useState } from 'react'
+import { SortableList, Text } from '../../src'
+import { Demo } from '../components/Demo'
+import { PropsTable } from '../components/PropsTable'
+import { Section } from '../components/Section'
+
+const INITIAL = [
+  { key: 'pin', label: '常用推荐', meta: 12 },
+  { key: 'movie', label: '推荐电影', meta: 12 },
+  { key: 'ces', label: 'CES 现场报道', meta: 12 },
+  { key: 'tools', label: '开发工具', meta: 5 },
+]
+
+export function Sortables() {
+  const [items, setItems] = useState(INITIAL)
+  const [activeKey, setActiveKey] = useState('pin')
+
+  const reorder = (keys: string[]) => {
+    setItems((current) =>
+      keys
+        .map((key) => current.find((item) => item.key === key))
+        .filter((item): item is (typeof INITIAL)[number] => item !== undefined),
+    )
+  }
+
+  return (
+    <Section
+      id="sortable"
+      title="排序列表"
+      api="SortableList"
+      desc="一列可拖拽排序的条目：拖动改顺序、点击选中，顺序变化通过 onReorder 交回页面。只做鼠标拖拽，需要键盘排序时在外面配上移/下移按钮。"
+    >
+      <Demo
+        title="基础用法"
+        desc="拖住左侧手柄上下移动，松手后把新的 key 顺序交给 onReorder；点击选中项会高亮。"
+        layout="column"
+        code={`const [items, setItems] = useState(INITIAL)
+
+<SortableList
+  items={items}
+  activeKey={activeKey}
+  onSelect={setActiveKey}
+  onReorder={(keys) => setItems(reorder(keys))}
+/>`}
+      >
+        <div style={{ width: 280 }}>
+          <SortableList
+            items={items}
+            activeKey={activeKey}
+            onSelect={setActiveKey}
+            onReorder={reorder}
+          />
+        </div>
+      </Demo>
+
+      <Demo
+        title="空态"
+        desc="items 为空时渲染 emptyText；不传就不显示空态。"
+        layout="column"
+        code={`<SortableList items={[]} onSelect={() => {}} onReorder={() => {}} emptyText="还没有卡片" />`}
+      >
+        <div style={{ width: 280 }}>
+          <SortableList
+            items={[]}
+            onSelect={() => undefined}
+            onReorder={() => undefined}
+            emptyText="还没有卡片"
+          />
+        </div>
+      </Demo>
+
+      <PropsTable
+        rows={[
+          { name: 'items', type: 'SortableItem[]', required: true, desc: '{ key, label, meta? }' },
+          { name: 'onSelect', type: '(key: string) => void', required: true, desc: '点击选中' },
+          { name: 'onReorder', type: '(keys: string[]) => void', required: true, desc: '拖拽后的完整顺序' },
+          { name: 'activeKey', type: 'string', desc: '当前选中项' },
+          { name: 'disabled', type: 'boolean', defaultValue: 'false', desc: '排序保存中，禁止再次拖动' },
+          { name: 'emptyText', type: 'ReactNode', desc: '空态文案，不传则不渲染' },
+        ]}
+      />
+
+      <Text size="sm" tone="muted">
+        说明：拖拽只在条目之间生效；松手时若顺序没变不会触发 onReorder。
+      </Text>
+    </Section>
+  )
+}