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

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

@@ -66,8 +66,8 @@ describe("CheckerRegistry", () => {
     const second = createDefaultCheckerRegistry();
     first.register(createChecker("custom"));
 
-    expect(first.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "ping", "custom"]);
-    expect(second.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "ping"]);
+    expect(first.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "ping", "udp", "custom"]);
+    expect(second.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "ping", "udp"]);
     expect(
       first.definitions.every(
         (checker) => checker.schemas.config && checker.schemas.defaults && checker.schemas.expect,