refactor: 引入 Checker 统一接口与 Runner 抽象机制

定义 Checker 接口（resolve/execute/serialize）和 CheckerRegistry
注册中心，消除 engine/config-loader/store 中硬编码类型分支。
按 checker 类型分子包（runner/http/、runner/command/），提取
共享 expect 到 runner/shared/。超时控制通过引擎注入 AbortSignal。
CheckFailure.phase 从联合类型改为 string。配置校验下沉到各
Checker.resolve() 内部。

新增 checker-runner-abstraction spec，更新 DEVELOPMENT.md。

src/server/checker/expect/failure.ts

@@ -1,34 +0,0 @@
-import type { CheckFailure } from "../types";
-
-export function truncateActual(value: unknown, maxLen = 200): unknown {
-  if (value === undefined || value === null) return value;
-  const str = String(value);
-  if (str.length <= maxLen) return value;
-  return str.slice(0, maxLen) + "...";
-}
-
-export function mismatchFailure(
-  phase: CheckFailure["phase"],
-  path: string,
-  expected: unknown,
-  actual: unknown,
-  message: string,
-): CheckFailure {
-  return {
-    kind: "mismatch",
-    phase,
-    path,
-    expected,
-    actual: truncateActual(actual),
-    message,
-  };
-}
-
-export function errorFailure(phase: CheckFailure["phase"], path: string, message: string): CheckFailure {
-  return {
-    kind: "error",
-    phase,
-    path,
-    message,
-  };
-}