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

tests/server/checker/runner/shared/body.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { checkBodyExpect } from "../../../../../src/server/checker/runner/shared/body";
+import { checkBodyExpect } from "../../../../../src/server/checker/runner/http/body";
 
 describe("checkBodyExpect (BodyRule[])", () => {
   test("无规则返回匹配成功", () => {

tests/server/checker/runner/shared/duration.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { checkDuration } from "../../../../../src/server/checker/runner/shared/duration";
+import { checkDuration } from "../../../../../src/server/checker/expect/duration";
 
 describe("checkDuration", () => {
   test("未配置 maxDurationMs 返回匹配成功", () => {

tests/server/checker/runner/shared/failure.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { errorFailure, mismatchFailure, truncateActual } from "../../../../../src/server/checker/runner/shared/failure";
+import { errorFailure, mismatchFailure, truncateActual } from "../../../../../src/server/checker/expect/failure";
 
 describe("truncateActual", () => {
   test("短字符串不截断", () => {

tests/server/checker/runner/shared/operator.test.ts

@@ -1,10 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import {
-  applyOperator,
-  checkExpectValue,
-  evaluateJsonPath,
-} from "../../../../../src/server/checker/runner/shared/operator";
+import { applyOperator, checkExpectValue, evaluateJsonPath } from "../../../../../src/server/checker/expect/operator";
 
 describe("evaluateJsonPath", () => {
   const obj = {

tests/server/checker/runner/shared/text.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { checkTextRules } from "../../../../../src/server/checker/runner/shared/text";
+import { checkTextRules } from "../../../../../src/server/checker/runner/command/text";
 
 describe("checkTextRules", () => {
   test("无规则返回匹配成功", () => {