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/cmd/runner.test.ts

@@ -45,14 +45,14 @@ describe("CommandChecker", () => {
   test("exitCode=0 成功", async () => {
     const result = await checker.execute(makeTarget({ args: ["-e", "process.exit(0)"], exec: "bun" }), makeCtx());
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("exitCode=0");
+    expect(result.observation).toMatchObject({ exitCode: 0 });
     expect(result.failure).toBeNull();
   });
 
   test("exitCode=1 不匹配默认 [0]", async () => {
     const result = await checker.execute(makeTarget({ args: ["-e", "process.exit(1)"], exec: "bun" }), makeCtx());
     expect(result.matched).toBe(false);
-    expect(result.statusDetail).toBe("exitCode=1");
+    expect(result.observation).toMatchObject({ exitCode: 1 });
     expect(result.failure!.phase).toBe("exitCode");
   });
 
@@ -62,7 +62,7 @@ describe("CommandChecker", () => {
       makeCtx(),
     );
     expect(result.matched).toBe(true);
-    expect(result.statusDetail).toBe("exitCode=1");
+    expect(result.observation).toMatchObject({ exitCode: 1 });
   });
 
   test("命令不存在返回 spawn 错误", async () => {
@@ -79,6 +79,7 @@ describe("CommandChecker", () => {
     );
     expect(result.matched).toBe(false);
     expect(result.failure!.message).toContain("超时");
+    expect(result.observation?.["error"]).toContain("超时");
   });
 
   test("stdout 输出捕获", async () => {
@@ -130,6 +131,7 @@ describe("CommandChecker", () => {
     );
     expect(result.matched).toBe(false);
     expect(result.failure!.message).toContain("超过限制");
+    expect(result.observation?.["error"]).toContain("超过限制");
   });
 
   test("durationMs 非空", async () => {