feat: 新增 UDP checker，支持自定义 payload 请求-响应探测与断言

基于 Bun connected UDP socket 实现通用 UDP 拨测能力：
- 支持 text/hex/base64 payload 编码与独立 responseEncoding 响应视图
- 支持 responded、response、responseSize、sourceHost、sourcePort、maxDurationMs 专属 expect
- 单 datagram 发送，仅断言首个 UDP 响应 datagram
- 通过 maxResponseBytes 和 flags.truncated 进行响应大小限制与截断保护
- payload 可选，省略时发送空 datagram
- 自包含模块结构（types/schema/validate/expect/encoding/execute）
- 新增 741 tests（含 unit、execute 集成、expect 和编码 roundtrip），全部通过

src/server/checker/runner/udp/schema.ts

@@ -0,0 +1,38 @@
+import { Type } from "@sinclair/typebox";
+
+import type { CheckerSchemas } from "../types";
+
+import { createPureOperatorSchema, createTextRulesSchema, sizeSchema } from "../../schema/fragments";
+
+export const udpCheckerSchemas: CheckerSchemas = {
+  config: Type.Object(
+    {
+      encoding: Type.Optional(Type.Union([Type.Literal("text"), Type.Literal("hex"), Type.Literal("base64")])),
+      host: Type.String({ minLength: 1 }),
+      maxResponseBytes: Type.Optional(sizeSchema),
+      payload: Type.Optional(Type.String()),
+      port: Type.Integer({ maximum: 65535, minimum: 1 }),
+      responseEncoding: Type.Optional(Type.Union([Type.Literal("text"), Type.Literal("hex"), Type.Literal("base64")])),
+    },
+    { additionalProperties: false },
+  ),
+  defaults: Type.Object(
+    {
+      encoding: Type.Optional(Type.Union([Type.Literal("text"), Type.Literal("hex"), Type.Literal("base64")])),
+      maxResponseBytes: Type.Optional(sizeSchema),
+      responseEncoding: Type.Optional(Type.Union([Type.Literal("text"), Type.Literal("hex"), Type.Literal("base64")])),
+    },
+    { additionalProperties: false },
+  ),
+  expect: Type.Object(
+    {
+      maxDurationMs: Type.Optional(Type.Number({ minimum: 0 })),
+      responded: Type.Optional(Type.Boolean()),
+      response: Type.Optional(createTextRulesSchema()),
+      responseSize: Type.Optional(createPureOperatorSchema()),
+      sourceHost: Type.Optional(createPureOperatorSchema()),
+      sourcePort: Type.Optional(createPureOperatorSchema()),
+    },
+    { additionalProperties: false },
+  ),
+};