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。

tests/server/checker/runner/registry.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test";
+import { CheckerRegistry } from "../../../../src/server/checker/runner/registry";
+import type { Checker } from "../../../../src/server/checker/runner/types";
+
+function createChecker(type: string): Checker {
+  return {
+    type,
+    resolve: () => ({}) as any,
+    execute: () => Promise.resolve({} as any),
+    serialize: () => ({ target: "", config: "" }),
+  };
+}
+
+describe("CheckerRegistry", () => {
+  test("注册并获取 Checker", () => {
+    const registry = new CheckerRegistry();
+    const checker = createChecker("http");
+    registry.register(checker);
+    expect(registry.get("http")).toBe(checker);
+  });
+
+  test("获取未注册的 type 抛出错误", () => {
+    const registry = new CheckerRegistry();
+    expect(() => registry.get("unknown")).toThrow("不支持的 probe type");
+  });
+
+  test("重复注册同一 type 抛出错误", () => {
+    const registry = new CheckerRegistry();
+    registry.register(createChecker("http"));
+    expect(() => registry.register(createChecker("http"))).toThrow("已注册");
+  });
+
+  test("查询支持的 type 列表", () => {
+    const registry = new CheckerRegistry();
+    registry.register(createChecker("http"));
+    registry.register(createChecker("command"));
+    expect(registry.supportedTypes).toEqual(["http", "command"]);
+  });
+});