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/tcp/execute.test.ts

@@ -127,7 +127,7 @@ describe("TcpChecker execute", () => {
     expect(result.matched).toBe(true);
     expect(result.failure).toBeNull();
     expect(result.durationMs).toBeGreaterThanOrEqual(0);
-    expect(result.statusDetail).toMatch(/^connected in \d+ms$/);
+    expect(result.observation).toMatchObject({ connected: true });
   });
 
   test("TCP 连接失败", async () => {
@@ -145,7 +145,7 @@ describe("TcpChecker execute", () => {
     );
     expect(result.matched).toBe(true);
     expect(result.failure).toBeNull();
-    expect(result.statusDetail).toBeTruthy();
+    expect(result.observation).toMatchObject({ connected: false });
   });
 
   test("期望端口不可达但连接成功", async () => {
@@ -171,8 +171,11 @@ describe("TcpChecker execute", () => {
       makeCtx(),
     );
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toContain("banner:");
-    expect(result.statusDetail).toContain("220 smtp.example.com ESMTP");
+    const obs = result.observation!;
+    expect(obs).toMatchObject({
+      connected: true,
+    });
+    expect(obs["banner"]).toContain("220 smtp.example.com ESMTP");
   });
 
   test("banner operator 校验通过", async () => {
@@ -206,7 +209,7 @@ describe("TcpChecker execute", () => {
       makeCtx(),
     );
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).not.toContain("banner:");
+    expect(result.observation?.["banner"]).toBeFalsy();
   });
 
   test("banner 超时空字符串继续执行", async () => {