feat: 汎用プラグインBox生成システム実装とnyash.toml v2対応準備

- GenericPluginBoxを実装し、任意のプラグインBoxを動的に生成可能に
- FileBox決め打ちコードを削除（設計思想違反の解消）
- CURRENT_TASK.mdを更新し、nyash.toml v2対応の必要性を明確化
- 問題: プラグインテスターとNyash本体が古い単一Box型形式のまま

次のステップ：
1. nyash.tomlをv2形式（マルチBox型）に更新
2. プラグインテスターをv2対応に
3. Nyash本体のレジストリをv2対応に

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/bin/test_plugin_loader_v2.rs

@@ -0,0 +1,57 @@
+//! Test program for multi-box plugin loader
+
+use nyash_rust::config::NyashConfigV2;
+use nyash_rust::runtime::get_global_loader;
+
+fn main() {
+    env_logger::init();
+    
+    println!("=== Multi-Box Plugin Loader Test ===\n");
+    
+    // Load configuration
+    let config_path = "test_nyash_v2.toml";
+    println!("Loading configuration from: {}", config_path);
+    
+    let config = match NyashConfigV2::from_file(config_path) {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            eprintln!("Failed to load config: {}", e);
+            return;
+        }
+    };
+    
+    println!("Configuration loaded successfully!");
+    println!("Is v2 format: {}", config.is_v2_format());
+    
+    if let Some(libs) = &config.plugins.libraries {
+        println!("\nLibraries found:");
+        for (name, lib) in libs {
+            println!("  {} -> {}", name, lib.plugin_path);
+            println!("    Provides: {:?}", lib.provides);
+        }
+    }
+    
+    // Load plugins
+    println!("\nLoading plugins...");
+    let loader = get_global_loader();
+    
+    match loader.load_from_config(&config) {
+        Ok(_) => println!("Plugins loaded successfully!"),
+        Err(e) => {
+            eprintln!("Failed to load plugins: {}", e);
+            return;
+        }
+    }
+    
+    // Test box type resolution
+    println!("\nTesting box type resolution:");
+    for box_type in ["TestBoxA", "TestBoxB", "UnknownBox"] {
+        if let Some(lib_name) = loader.get_library_for_box(box_type) {
+            println!("  {} -> library: {}", box_type, lib_name);
+        } else {
+            println!("  {} -> not found", box_type);
+        }
+    }
+    
+    println!("\nTest completed!");
+}