chore(backlog): 记录 g51 完成

.ua/backlog.json

@@ -764,6 +764,14 @@
       "acceptance": "新增 datePicker（单月网格、翻月、今天、min/max 闭区间、←→↑↓/PageUp/PageDown/Enter、Esc 入栈、value 信号）；manifest/docs/demo/README/计数同步；npm run check 全绿",
       "status": "done",
       "notes": "自补：表单组件补缺；跨月自动翻页，漫游 tabindex 只有焦点日可 Tab 进入"
+    },
+    {
+      "id": "g51",
+      "title": "新增 backTop 返回顶部按钮",
+      "motivation": "长文档/长表格页面缺少回顶入口，用户只能拖滚动条；进度环还能给出「还剩多少没看」的定位感",
+      "acceptance": "新增 backTop（阈值浮现、滚动进度环、平滑回顶且尊重 reduced-motion、支持内滚容器 target）；manifest/docs/demo/README/计数同步；npm run check 全绿",
+      "status": "done",
+      "notes": "自补：组件补充；进度环用 conic-gradient 内联写角度，不引入新令牌"
     }
   ]
 }

.ua/iterations.log

@@ -95,3 +95,4 @@
 {"ts":"2026-10-26T12:11:45+08:00","id":"g48","result":"done","commit":"d7bfabb","summary":"新增 splitter 可拖拽分栏：指针拖拽+方向键微调+双击复位+min 夹逼+横纵两向，组件数 46→47"}
 {"ts":"2026-10-26T12:12:56+08:00","id":"g49","result":"done","commit":"cb8a9ea","summary":"新增 autoComplete 自动补全：输入即过滤、↑↓ 环形高亮、Enter 选中、Esc 入栈、options 支持函数式远程联想，组件数 47→48"}
 {"ts":"2026-10-26T12:14:00+08:00","id":"g50","result":"done","commit":"e270e09","summary":"新增 datePicker 日期选择：单月网格+翻月/今天+min-max 闭区间+逐日逐周逐月键盘导航，组件数 48→49"}
+{"ts":"2026-10-26T12:15:04+08:00","id":"g51","result":"done","commit":"b93b6dd","summary":"新增 backTop 返回顶部：阈值浮现+conic 进度环+平滑回顶（尊重 reduced-motion）+内滚容器支持，组件数 49→50"}