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/llm/execute.test.ts

@@ -111,10 +111,10 @@ describe("LlmChecker execute - 非流式", () => {
     const result = await checker.execute(makeTarget(), makeCtx());
     expect(result.matched).toBe(true);
     expect(result.failure).toBeNull();
-    expect(result.statusDetail).toContain("openai");
-    expect(result.statusDetail).toContain("http");
-    expect(result.statusDetail).toContain("200");
-    expect(result.statusDetail).toContain("finish=stop");
+    expect(result.observation).toMatchObject({ provider: "openai" });
+    expect(result.observation).toMatchObject({ mode: "http" });
+    expect(result.observation).toMatchObject({ http: { status: 200 } });
+    expect(result.observation).toMatchObject({ finishReason: "stop" });
   });
 
   test("status expect 不匹配", async () => {
@@ -168,12 +168,11 @@ describe("LlmChecker execute - 非流式", () => {
     expect(result.failure?.phase).toBe("request");
   });
 
-  test("statusDetail 包含 output 长度和 usage", async () => {
+  test("observation 包含 output 长度和 usage", async () => {
     const result = await checker.execute(makeTarget(), makeCtx());
-    expect(result.statusDetail).toContain("output=");
-    expect(result.statusDetail).toContain("chars");
-    expect(result.statusDetail).toContain("usage=");
-    expect(result.statusDetail).toContain("tokens");
+    expect(result.observation).toHaveProperty("outputPreview");
+    expect(result.observation).toHaveProperty("outputLength");
+    expect(result.observation).toHaveProperty("usage");
   });
 
   test("无文本输出且配置 output expect 失败", async () => {