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>

src/main.rs

@@ -1,5 +1,6 @@
 use anyhow::Result;
 use showen_v2::core::config::AppConfig;
+use showen_v2::core::plugin_loader::PluginLoader;
 use showen_v2::core::service_manager::ServiceManager;
 use showen_v2::plugins::{
     ble::BlePlugin, http::HttpPlugin, screen::ScreenPlugin, video::VideoPlugin, wifi::WifiPlugin,
@@ -64,6 +65,46 @@ fn main() -> Result<()> {
     manager.register(Box::new(HttpPlugin::new()));
     println!("  ✓ HttpPlugin");
 
+    // 加载动态插件
+    let plugin_store = std::path::Path::new("plugin_store");
+    if plugin_store.exists() {
+        println!("扫描动态插件...");
+        let loader = PluginLoader::new(plugin_store);
+        match loader.load_registry() {
+            Ok(registry) => {
+                for (plugin_id, entry) in &registry.plugins {
+                    if !entry.enabled {
+                        println!("  - {plugin_id} (禁用)");
+                        continue;
+                    }
+
+                    match loader.load_plugin(plugin_id, Some(&entry.active_version)) {
+                        Ok((plugin, manifest)) => {
+                            manager.register_dynamic(
+                                Box::new(plugin),
+                                manifest.error_policy,
+                                entry.max_errors,
+                            );
+                            println!(
+                                "  ✓ {} v{} (动态)",
+                                plugin_id, entry.active_version
+                            );
+                        }
+                        Err(e) => {
+                            eprintln!(
+                                "  ✗ {} v{} 加载失败: {e}",
+                                plugin_id, entry.active_version
+                            );
+                        }
+                    }
+                }
+            }
+            Err(e) => {
+                eprintln!("读取插件注册表失败: {e}");
+            }
+        }
+    }
+
     // 设置 Ctrl+C 信号处理
     let running = Arc::new(AtomicBool::new(true));
     let r = running.clone();