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

src/server/checker/engine.ts

@@ -73,10 +73,11 @@ export class ProbeEngine {
         console.warn("探针执行失败:", result.reason);
         if (!target) continue;
         this.writeResult({
+          detail: null,
           durationMs: null,
           failure: errorFailure("internal", "engine", formatReason(result.reason)),
           matched: false,
-          statusDetail: null,
+          observation: null,
           targetId: target.id,
           timestamp: new Date().toISOString(),
         });
@@ -110,7 +111,7 @@ export class ProbeEngine {
       durationMs: result.durationMs,
       failure: result.failure,
       matched: result.matched,
-      statusDetail: result.statusDetail,
+      observation: result.observation ?? null,
       targetId: result.targetId,
       timestamp: result.timestamp,
     });