feat: 实现 NetworkLoop::run — Server/Client renet 主循环

添加 renet2_netcode 依赖，使用 renet2 + renet2_netcode + renetcode2
三 crate 架构实现完整网络循环。Server 端监听 UDP 端口并通过
channel 广播游戏消息，Client 端连接并双工通信。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/Cargo.toml

@@ -11,4 +11,5 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 reqwest = { version = "0.12", features = ["json", "blocking"] }
 renet2 = "0.15"
+renet2_netcode = "0.15"
 bincode = "1"