fix(ai): 折叠态只显示一句话，那句话得说清是哪个指标

精简版的卡片正文就是第一条 highlight，标题不渲染。于是趋势页折叠起来是
「395 天内由 4376.0分 到 4905.0分（改善）」——什么的 4376 分？身体成分、
运动详情同样。

每个 builder 的第一条 detail 现在都自带主语，并加了不变量测试：detail 必须
包含它自己的 title（纯汇总性标题除外）。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/services/coach.py

@@ -495,6 +495,10 @@ def rule_scope_insight(context):
     return {
         # The first highlight leads and is then dropped from the list: showing
         # it in both places printed the same sentence twice.
+        #
+        # This is why every builder's first `detail` has to read on its own —
+        # here the `title` beside it is not rendered, and "395 天内由 4376 到
+        # 4905（改善）" without naming 耐力分 is a sentence about nothing.
         "headline": highlights[0]["detail"] if highlights else None,
         "points": [dict(h) for h in highlights[1:6]],
         "actions": [],