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>

gui/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "gobang-gui"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+authors.workspace = true
+repository.workspace = true
+
+[dependencies]
+gobang-core = { path = "../core" }

gui/src/lib.rs

@@ -0,0 +1 @@
+// Gobang GUI — 占位符，Task 10 中实现