feat: 结构化 observation 替代 statusDetail，API 层动态构造 detail

- CheckResult: statusDetail -> observation (持久化) + detail (API 动态派生)
- 存储: status_detail 列 -> observation TEXT (JSON)
- CheckerDefinition: 新增 buildDetail(observation) 方法
- 各 checker 返回结构化 observation，API 层通过 registry 调用 buildDetail
- HTTP: bodyPreview 在 status/header 失败时也提前采集
- UDP: observation 包含 durationMs，未响应归为 error failure
- CMD: 超时/输出超限时保留已收集 observation
- TCP: connectTimeMs 仅含连接建立耗时，不含 banner 等待
- 新增 buildDetail 单测和 mapCheckResult 覆盖测试
- 同步 openspec 主规范，归档 checker-observation 变更

src/web/components/OverviewTab.tsx

@@ -37,7 +37,7 @@ export function OverviewTab({ metricsData, metricsLoading, target }: OverviewTab
             content: target.latestCheck ? new Date(target.latestCheck.timestamp).toLocaleString("zh-CN") : "-",
             label: "最新检查时间",
           },
-          { content: target.latestCheck?.statusDetail ?? "-", label: "状态详情" },
+          { content: target.latestCheck?.detail ?? "-", label: "状态详情" },
           { content: target.description ?? "", label: "描述", span: 2 },
         ]}
       />

src/web/constants/history-table-columns.tsx

@@ -27,10 +27,10 @@ export const HISTORY_COLUMNS: Array<PrimaryTableCol<CheckResult>> = [
   },
   {
     cell: ({ row }: PrimaryTableCellParams<CheckResult>) => {
-      const parts = [row.statusDetail, row.failure?.message].filter(Boolean);
+      const parts = [row.detail, row.failure?.message].filter(Boolean);
       return parts.length > 0 ? parts.join("：") : "-";
     },
-    colKey: "statusDetail",
+    colKey: "detail",
     title: "详情",
   },
 ];