chore(backlog): 记录 g174 完成

.ua/backlog.json

@@ -1748,6 +1748,14 @@
       "acceptance": "upload 对 image/* 文件渲染 28px 缩略图（object-fit: cover），对象 URL 按 File 缓存、文件离场即 revoke；非图片保持文件名 + 体积；manifest 步骤、llms-full 同步；npm run check 全绿。",
       "notes": "",
       "status": "done"
+    },
+    {
+      "id": "g174",
+      "title": "时间线支持双侧交替",
+      "motivation": "里程碑 / 项目历程这类内容用单侧时间线时，长正文会把右侧挤成一条窄柱；做左右交替得自己包两层栅格，pending 虚节点还容易错位。",
+      "acceptance": "timeline 的 mode 增加 alternate：轴线居中、条目按奇偶分列左右、圆点贴中线、窄屏自动退回单侧（pending 虚节点参与交替计数）；演示页加里程碑实例；manifest 参数类型与步骤、llms-full 同步；npm run check 全绿。",
+      "notes": "",
+      "status": "done"
     }
   ]
 }

.ua/iterations.log

@@ -218,3 +218,4 @@
 {"ts":"2026-10-26T16:17:49+08:00","id":"g171","result":"done","commit":"6a605ba","summary":"刷新 CHANGELOG 至 400 提交，补 llms.txt 的配方 11 目录行"}
 {"ts":"2026-10-26T16:19:32+08:00","id":"g172","result":"done","commit":"250601d","summary":"ua list 新增 --json（包 + 组件 id 结构化输出），CLI 清单与 AGENTS 同步"}
 {"ts":"2026-10-26T16:20:30+08:00","id":"g173","result":"done","commit":"ea4e781","summary":"upload 图片文件带 28px 缩略图，对象 URL 随文件离场 revoke"}
+{"ts":"2026-10-26T16:21:48+08:00","id":"g174","result":"done","commit":"2316445","summary":"timeline 新增 mode=alternate（轴线居中、奇偶分列、窄屏回落单侧）"}