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/icmp/execute.test.ts

@@ -54,7 +54,13 @@ rtt min/avg/max/mdev = 1.234/2.345/3.456/0.567 ms`);
     const result = await checker.execute(makeTarget(), makeCtx());
     expect(result.matched).toBe(true);
     expect(result.failure).toBeNull();
-    expect(result.statusDetail).toBe("alive, avg 2.345ms, loss 0% (3/3)");
+    expect(result.observation).toMatchObject({
+      alive: true,
+      avgLatencyMs: 2.345,
+      packetLoss: 0,
+      received: 3,
+      transmitted: 3,
+    });
     expect(calls[0]).toContain("ping");
   });
 
@@ -66,7 +72,7 @@ rtt min/avg/max/mdev = 1.234/2.345/3.456/0.567 ms`);
     );
     expect(result.matched).toBe(false);
     expect(result.failure?.phase).toBe("alive");
-    expect(result.statusDetail).toBe("unreachable (0/3 received)");
+    expect(result.observation).toMatchObject({ alive: false, received: 0, transmitted: 3 });
   });
 
   test("反向 alive 断言通过", async () => {
@@ -81,7 +87,7 @@ rtt min/avg/max/mdev = 1.234/156.000/340.000/0.567 ms`);
     const result = await checker.execute(makeTarget({ expect: { packetLossPercent: { lte: 10 } } }), makeCtx());
     expect(result.matched).toBe(false);
     expect(result.failure?.phase).toBe("packetLoss");
-    expect(result.statusDetail).toContain("max 340ms");
+    expect(result.observation).toMatchObject({ alive: true, maxLatencyMs: 340 });
   });
 
   test("解析失败返回结构化错误", async () => {
@@ -98,7 +104,7 @@ rtt min/avg/max/mdev = 1.234/156.000/340.000/0.567 ms`);
     const result = await checker.execute(makeTarget(), makeCtx());
     expect(result.matched).toBe(false);
     expect(result.failure?.message).toContain("ping 命令不可用");
-    expect(result.statusDetail).toBe("ping command not found");
+    expect(result.observation).toBeNull();
   });
 
   test("预 abort 返回超时错误", async () => {