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

tests/server/checker/runner/db/execute.test.ts

@@ -34,14 +34,14 @@ describe("DbChecker", () => {
   test("无 query 时仅测试连接成功", async () => {
     const result = await checker.execute(makeTarget({}), makeCtx());
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("connected");
+    expect(result.observation).toMatchObject({ connected: true });
     expect(result.failure).toBeNull();
   });
 
   test("执行查询成功", async () => {
     const result = await checker.execute(makeTarget({ query: "SELECT 1 as num, 'hello' as str" }), makeCtx());
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("1 rows");
+    expect(result.observation).toMatchObject({ connected: true, rowCount: 1 });
     expect(result.durationMs).toBeGreaterThanOrEqual(0);
   });
 
@@ -51,13 +51,13 @@ describe("DbChecker", () => {
       makeCtx(),
     );
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("3 rows");
+    expect(result.observation).toMatchObject({ connected: true, rowCount: 3 });
   });
 
   test("查询返回空结果", async () => {
     const result = await checker.execute(makeTarget({ query: "SELECT 1 as n WHERE 1=0" }), makeCtx());
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("0 rows");
+    expect(result.observation).toMatchObject({ connected: true, rowCount: 0 });
   });
 
   test("连接失败返回 connect phase 错误", async () => {