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/server/helpers.ts

@@ -1,6 +1,8 @@
 import type { ApiErrorResponse, CheckFailure, CheckResult, HealthResponse, RuntimeMode } from "../shared/api";
 import type { StoredCheckResult } from "./checker/types";
 
+import { checkerRegistry } from "./checker/runner";
+
 export function createApiError(error: string, status: number): ApiErrorResponse {
   return { error, status };
 }
@@ -45,7 +47,7 @@ export function jsonResponse(
   });
 }
 
-export function mapCheckResult(row: StoredCheckResult): CheckResult {
+export function mapCheckResult(row: StoredCheckResult, type: string): CheckResult {
   let failure: CheckFailure | null = null;
   if (row.failure) {
     try {
@@ -56,11 +58,32 @@ export function mapCheckResult(row: StoredCheckResult): CheckResult {
     }
   }
 
+  let observation: null | Record<string, unknown> = null;
+  if (row.observation) {
+    try {
+      observation = JSON.parse(row.observation) as Record<string, unknown>;
+    } catch {
+      console.warn(`无法解析 observation 数据: target_id=${row.target_id}, timestamp=${row.timestamp}`);
+      observation = null;
+    }
+  }
+
+  let detail: null | string = null;
+  if (observation !== null) {
+    try {
+      const checker = checkerRegistry.get(type);
+      detail = checker.buildDetail(observation);
+    } catch {
+      detail = null;
+    }
+  }
+
   return {
+    detail,
     durationMs: row.duration_ms,
     failure,
     matched: row.matched === 1,
-    statusDetail: row.status_detail,
+    observation,
     timestamp: row.timestamp,
   };
 }