refactor: 将 checker normalize 职责下沉到各 runner 目录

- 新增 CheckerDefinition.normalize 必需方法，typecheck 兜底遗漏实现
- 新增 expect/normalize.ts 共享 helper（compactExpect、normalizeValue、
  normalizeContent、normalizeKeyed）
- 为 HTTP、Cmd、DB、TCP、UDP、ICMP、LLM、WS、DNS 各新增独立 normalize.ts
- 简化 normalizer.ts：删除所有 checker type switch，改为 registry 委托
- 修复 DNS authoring 简写 bug：durationMs、valueCount、result 等字段
  现可通过完整加载链路
- 新增 DNS 回归测试和 registry 级合同测试
- 更新 docs/development/checker.md：补充 normalize 规范、文件结构、
  测试要求和 checklist

src/server/checker/runner/dns/execute.ts

@@ -25,6 +25,7 @@ import {
   checkTtlMin,
   checkValueCount,
 } from "./expect";
+import { normalizeTargetExpect } from "./normalize";
 import { dnsCheckerSchemas } from "./schema";
 import { queryDns } from "./transport";
 import { validateDnsConfig } from "./validate";
@@ -83,6 +84,10 @@ export class DnsChecker implements CheckerDefinition<ResolvedDnsTarget> {
     }
   }
 
+  normalize(target: RawTargetConfig): RawTargetConfig {
+    return normalizeTargetExpect(target);
+  }
+
   resolve(target: RawTargetConfig, context: ResolveContext): ResolvedDnsTarget {
     const dns = target["dns"] as DnsServerConfig | DnsSystemConfig;