feat: 初始化 Cargo workspace + core crate 脚手架

创建 Rust 工作区，包含 core（纯游戏逻辑）和 gui（Tauri 占位）两个 crate。

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

core/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "gobang-core"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+authors.workspace = true
+repository.workspace = true
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+renet = "2"
+reqwest = { version = "0.12", features = ["json", "blocking"] }
+rand = "0.8"

core/src/lib.rs

@@ -0,0 +1 @@
+// Gobang core library — 纯游戏逻辑，零 GUI 依赖