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/engine.test.ts

@@ -2,6 +2,16 @@ import { describe, expect, test } from "bun:test";
 import { ProbeEngine } from "../../../src/server/checker/engine";
 import type { ProbeStore } from "../../../src/server/checker/store";
 import type { ResolvedCommandTarget, ResolvedHttpTarget, ResolvedTarget } from "../../../src/server/checker/types";
+import { checkerRegistry } from "../../../src/server/checker/runner";
+import { HttpChecker } from "../../../src/server/checker/runner/http/runner";
+import { CommandChecker } from "../../../src/server/checker/runner/command/runner";
+
+function ensureRegistered() {
+  if (!checkerRegistry.supportedTypes.includes("http")) {
+    checkerRegistry.register(new HttpChecker());
+    checkerRegistry.register(new CommandChecker());
+  }
+}
 
 function createMockStore(targetNames: string[]) {
   let nextId = 1;
@@ -49,6 +59,7 @@ function makeCommandTarget(name: string, overrides?: Partial<ResolvedCommandTarg
 
 describe("ProbeEngine", () => {
   test("start/stop 不抛错", () => {
+    ensureRegistered();
     const mockStore = createMockStore(["test"]) as unknown as ProbeStore;
     const targets: ResolvedTarget[] = [makeCommandTarget("test")];
     const engine = new ProbeEngine(mockStore, targets);