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>

plugins/example-plugin/src/lib.rs

@@ -0,0 +1,72 @@
+//! 示例动态插件 — 展示如何使用 showen-plugin-sdk 编写插件
+//!
+//! 此插件仅打印日志，用于验证动态加载流程。
+
+use showen_plugin_sdk::{
+    export_plugin, Message, MessageSender, PluginInfo, ShowenPlugin,
+};
+
+pub struct ExamplePlugin {
+    sender: Option<MessageSender>,
+}
+
+impl ExamplePlugin {
+    pub fn new() -> Self {
+        Self { sender: None }
+    }
+}
+
+impl ShowenPlugin for ExamplePlugin {
+    fn info(&self) -> PluginInfo {
+        PluginInfo {
+            name: "example-plugin".to_string(),
+            version: "0.1.0".to_string(),
+            description: "示例动态插件".to_string(),
+            platform: "Any".to_string(),
+        }
+    }
+
+    fn init(&mut self, config_json: &str, sender: MessageSender) -> Result<(), String> {
+        eprintln!("[ExamplePlugin] init called, config length: {}", config_json.len());
+        self.sender = Some(sender);
+
+        // 通知主程序就绪
+        if let Some(sender) = &self.sender {
+            sender.send_to_manager(
+                "example-plugin",
+                Message::PluginReady("example-plugin".to_string()),
+            );
+        }
+
+        Ok(())
+    }
+
+    fn start(&mut self) -> Result<(), String> {
+        eprintln!("[ExamplePlugin] started");
+        Ok(())
+    }
+
+    fn handle_message(&mut self, message: Message) -> Result<(), String> {
+        match &message {
+            Message::Shutdown => {
+                eprintln!("[ExamplePlugin] received shutdown");
+            }
+            Message::Custom { kind, payload } => {
+                eprintln!("[ExamplePlugin] custom message: kind={kind}, payload={payload}");
+            }
+            _ => {
+                eprintln!("[ExamplePlugin] received message: {:?}", message);
+            }
+        }
+        Ok(())
+    }
+
+    fn stop(&mut self) -> Result<(), String> {
+        eprintln!("[ExamplePlugin] stopped");
+        self.sender = None;
+        Ok(())
+    }
+}
+
+// 导出 FFI 接口
+export_plugin!(ExamplePlugin, ExamplePlugin::new());