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/registry.test.ts

@@ -9,6 +9,7 @@ import { CheckerRegistry } from "../../../../src/server/checker/runner/registry"
 
 function createChecker(type: string): Checker {
   return {
+    buildDetail: () => null,
     configKey: type,
     execute: () => Promise.resolve<CheckResult>({} as unknown as CheckResult),
     resolve: () => ({}) as unknown as ResolvedTargetBase,