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/web/constants/target-table-columns.test.ts

@@ -110,10 +110,11 @@ describe("createTargetTableColumns", () => {
       colIndex: 6,
       row: makeTarget({
         latestCheck: {
+          detail: "200",
           durationMs: 12000,
           failure: null,
           matched: true,
-          statusDetail: "200",
+          observation: null,
           timestamp: "2026-01-01T00:00:00.000Z",
         },
       }),
@@ -139,10 +140,11 @@ describe("createTargetTableColumns", () => {
       colIndex: 6,
       row: makeTarget({
         latestCheck: {
+          detail: "200",
           durationMs: 123,
           failure: null,
           matched: true,
-          statusDetail: "200",
+          observation: null,
           timestamp: "2026-01-01T00:00:00.000Z",
         },
       }),