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/app.test.ts

@@ -80,7 +80,7 @@ describe("API 路由", () => {
       durationMs: 100,
       failure: null,
       matched: true,
-      statusDetail: "200 OK",
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: "2025-01-01T00:00:00.000Z",
     });
@@ -95,7 +95,7 @@ describe("API 路由", () => {
         phase: "status",
       },
       matched: false,
-      statusDetail: null,
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: "2025-01-01T00:10:00.000Z",
     });
@@ -110,7 +110,7 @@ describe("API 路由", () => {
         phase: "status",
       },
       matched: false,
-      statusDetail: null,
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: "2025-01-01T00:20:00.000Z",
     });
@@ -118,7 +118,7 @@ describe("API 路由", () => {
       durationMs: 200,
       failure: null,
       matched: true,
-      statusDetail: "200 OK",
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: "2025-01-01T00:40:00.000Z",
     });
@@ -126,7 +126,7 @@ describe("API 路由", () => {
       durationMs: 400,
       failure: null,
       matched: true,
-      statusDetail: "200 OK",
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: "2025-01-01T01:10:00.000Z",
     });
@@ -136,7 +136,7 @@ describe("API 路由", () => {
       durationMs: 120,
       failure: null,
       matched: true,
-      statusDetail: "200 OK",
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: new Date(now - 90 * 60 * 1000).toISOString(),
     });
@@ -151,7 +151,7 @@ describe("API 路由", () => {
         phase: "status",
       },
       matched: false,
-      statusDetail: null,
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: new Date(now - 60 * 60 * 1000).toISOString(),
     });
@@ -166,7 +166,7 @@ describe("API 路由", () => {
         phase: "status",
       },
       matched: false,
-      statusDetail: null,
+      observation: null,
       targetId: targets[0]!.id,
       timestamp: new Date(now - 30 * 60 * 1000).toISOString(),
     });
@@ -428,7 +428,7 @@ describe("API 路由", () => {
       durationMs: 100,
       failure: { kind: "error", message: "test", path: "$", phase: "body" },
       matched: false,
-      statusDetail: "200 OK",
+      observation: null,
       targetId: t1Id,
       timestamp: "2025-06-01T00:00:00.000Z",
     });