feat: 实现动态插件系统 (6阶段完成)

- 阶段1: 消息类型序列化 (Serialize/Deserialize, &'static str → String)
- 阶段2: FFI 边界类型 + Plugin SDK (plugin_abi, showen-plugin-sdk crate)
- 阶段3: PluginLoader + DynamicPlugin (libloading 动态加载 .so)
- 阶段4: 版本管理 + 错误策略 (VersionManager, PluginState, 自动回退)
- 阶段5: 远程仓库客户端 (HTTP 下载 + tar.gz 安装)
- 阶段6: 示例插件 + HTTP 管理 API + 全目录 README 文档

54/54 测试通过，0 warnings。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -1,3 +1,6 @@
+[workspace]
+members = [".", "plugin-sdk", "plugins/example-plugin"]
+
 [package]
 name = "showen_v2"
 version = "0.2.0"
@@ -21,3 +24,12 @@ futures-util = "0.3"
 # Linux 特有插件依赖
 dbus = "0.9"
 dbus-crossroads = "0.5"
+
+# 动态插件加载
+libloading = "0.8"
+
+# 远程插件仓库
+ureq = "2"
+flate2 = "1"
+tar = "0.4"
+semver = "1"