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/runner/llm/observation.ts

@@ -4,11 +4,15 @@ import type {
   LlmCheckObservation,
   LlmHttpMetadata,
   LlmMode,
+  LlmPersistedHttpMetadata,
+  LlmPersistedObservation,
   LlmProvider,
   LlmStreamObservation,
   LlmUsageObservation,
 } from "./types";
 
+import { LLM_HEADERS_MAX, LLM_OUTPUT_PREVIEW_MAX } from "./types";
+
 export function buildObservationFromApiCallError(
   error: APICallError,
   provider: LlmProvider,
@@ -129,3 +133,42 @@ export async function buildObservationFromStreamText(
     warnings,
   };
 }
+
+export function toPersistedObservation(obs: LlmCheckObservation): LlmPersistedObservation {
+  const outputText = obs.outputText;
+  const outputPreview =
+    outputText !== null
+      ? outputText.length <= LLM_OUTPUT_PREVIEW_MAX
+        ? outputText
+        : outputText.slice(0, LLM_OUTPUT_PREVIEW_MAX)
+      : null;
+  const outputLength = outputText !== null ? outputText.length : null;
+
+  const http: LlmPersistedHttpMetadata | null = obs.http
+    ? {
+        headers: truncateHeaders(obs.http.headers, LLM_HEADERS_MAX),
+        status: obs.http.status,
+        statusText: obs.http.statusText,
+      }
+    : null;
+
+  return {
+    finishReason: obs.finishReason,
+    http,
+    mode: obs.mode,
+    model: obs.model,
+    outputLength,
+    outputPreview,
+    provider: obs.provider,
+    rawFinishReason: obs.rawFinishReason,
+    stream: obs.stream,
+    usage: obs.usage,
+    warnings: obs.warnings,
+  };
+}
+
+function truncateHeaders(headers: Record<string, string>, maxCount: number): Record<string, string> {
+  const entries = Object.entries(headers);
+  if (entries.length <= maxCount) return headers;
+  return Object.fromEntries(entries.slice(0, maxCount));
+}