refactor: checker 模块内聚化 — 每个 checker 自包含于独立目录

将 checker 架构重构为完全内聚模式：每个 checker 目录包含自身的
types、schema、validate、execute、expect 和 index，新增 checker
只需创建一个目录并在 runner/index.ts 添加一行注册。

主要变更：
- runner/shared/ 拆分：断言基础设施迁入 checker/expect/，
  body.ts 迁入 http/，text.ts 迁入 command/
- config-contract/ 重命名为 schema/，schema.ts → builder.ts
- size.ts + parseDuration 合并为 utils.ts
- 顶层 types.ts 改为 base interface + index signature，
  checker 专属类型下沉到各自 types.ts
- runner/index.ts 改为显式数组注册模式
- 更新 DEVELOPMENT.md 项目结构和开发新 Checker 指南

src/server/checker/engine.ts

@@ -1,7 +1,7 @@
 import { groupBy, Semaphore } from "es-toolkit";
 
 import type { ProbeStore } from "./store";
-import type { CheckResult, ResolvedTarget } from "./types";
+import type { CheckResult, ResolvedTargetBase } from "./types";
 
 import { checkerRegistry } from "./runner";
 
@@ -9,10 +9,10 @@ export class ProbeEngine {
   private semaphore: Semaphore;
   private store: ProbeStore;
   private targetNameToId = new Map<string, number>();
-  private targets: ResolvedTarget[];
+  private targets: ResolvedTargetBase[];
   private timers: Array<ReturnType<typeof setInterval>> = [];
 
-  constructor(store: ProbeStore, targets: ResolvedTarget[], maxConcurrentChecks?: number) {
+  constructor(store: ProbeStore, targets: ResolvedTargetBase[], maxConcurrentChecks?: number) {
     this.store = store;
     this.targets = targets;
     this.semaphore = new Semaphore(maxConcurrentChecks ?? 20);
@@ -40,7 +40,7 @@ export class ProbeEngine {
     this.timers = [];
   }
 
-  private async probeGroup(targets: ResolvedTarget[]): Promise<void> {
+  private async probeGroup(targets: ResolvedTargetBase[]): Promise<void> {
     const results = await Promise.allSettled(
       targets.map(async (target) => {
         await this.semaphore.acquire();
@@ -68,7 +68,7 @@ export class ProbeEngine {
     }
   }
 
-  private async runCheck(target: ResolvedTarget): Promise<CheckResult> {
+  private async runCheck(target: ResolvedTargetBase): Promise<CheckResult> {
     const checker = checkerRegistry.get(target.type);
     const controller = new AbortController();
     const timeoutId = setTimeout(() => controller.abort(), target.timeoutMs);