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/tests/test_coach.py

@@ -1053,3 +1053,23 @@ class TestSupersede:
         statuses = {r["subject"]: r["status"] for r in analysis_svc.query_all(
             "SELECT subject, status FROM ai_jobs WHERE kind = 'trends'")}
         assert statuses.get("2026-08-01") == "running"
+
+
+class TestHighlightsReadAlone:
+    """The first highlight becomes the collapsed card's whole text, with no
+    title beside it — so it has to name what it is about."""
+
+    def test_every_first_highlight_names_its_subject(self, month):
+        for name in scopes.SCOPES:
+            built = scopes.build(month["id"], name)
+            if not built:
+                continue
+            first = built[1]["highlights"][0]
+            title = first["title"]
+            # Titles that are already summaries rather than names of a metric.
+            if title in ("整体", "当前预测") or title.startswith("近 "):
+                continue
+            assert title in first["detail"], (
+                f"{name}: 折叠态只显示 detail，但它没提到 {title!r}: "
+                f"{first['detail']!r}"
+            )