feat(phase-9.75g-0): Implement Day 4 plugin system infrastructure (80% complete)

Core plugin system components implemented:
- nyash.toml parser for plugin configuration
- BoxFactoryRegistry for unified Box creation management
- PluginBox proxy for FFI boundary abstraction
- Runtime module integration

Key features:
- Simple TOML parsing without external dependencies
- Transparent Box switching (builtin ↔ plugin)
- Everything is Box philosophy maintained
- Thread-safe design with RwLock

Remaining Day 4 tasks:
- FileBox FFI interface definition
- Dynamic plugin loading with libloading
- Plugin FileBox integration tests

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

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

src/runtime/box_registry.rs

@@ -0,0 +1,137 @@
+//! Boxファクトリレジストリ - Box生成の中央管理
+//! 
+//! ビルトインBoxとプラグインBoxを統一的に管理し、
+//! 透過的な置き換えを実現する
+
+use crate::box_trait::NyashBox;
+use crate::runtime::plugin_config::PluginConfig;
+use std::collections::HashMap;
+use std::sync::{Arc, RwLock};
+
+/// Box生成方法を表す列挙型
+pub enum BoxProvider {
+    /// ビルトイン実装（Rust関数）
+    Builtin(BoxConstructor),
+    
+    /// プラグイン実装（プラグイン名を保持）
+    Plugin(String),
+}
+
+/// ビルトインBoxのコンストラクタ関数型
+pub type BoxConstructor = fn(&[Box<dyn NyashBox>]) -> Result<Box<dyn NyashBox>, String>;
+
+/// Boxファクトリレジストリ
+pub struct BoxFactoryRegistry {
+    /// Box名 → プロバイダーのマッピング
+    providers: RwLock<HashMap<String, BoxProvider>>,
+}
+
+impl BoxFactoryRegistry {
+    /// 新しいレジストリを作成
+    pub fn new() -> Self {
+        Self {
+            providers: RwLock::new(HashMap::new()),
+        }
+    }
+    
+    /// ビルトインBoxを登録
+    pub fn register_builtin(&self, name: &str, constructor: BoxConstructor) {
+        let mut providers = self.providers.write().unwrap();
+        providers.insert(name.to_string(), BoxProvider::Builtin(constructor));
+    }
+    
+    /// プラグイン設定を適用（既存のビルトインを上書き）
+    pub fn apply_plugin_config(&self, config: &PluginConfig) {
+        let mut providers = self.providers.write().unwrap();
+        
+        for (box_name, plugin_name) in &config.plugins {
+            providers.insert(
+                box_name.clone(),
+                BoxProvider::Plugin(plugin_name.clone())
+            );
+        }
+    }
+    
+    /// Box名からプロバイダーを取得
+    pub fn get_provider(&self, name: &str) -> Option<BoxProvider> {
+        let providers = self.providers.read().unwrap();
+        providers.get(name).cloned()
+    }
+    
+    /// Boxを生成
+    pub fn create_box(&self, name: &str, args: &[Box<dyn NyashBox>]) -> Result<Box<dyn NyashBox>, String> {
+        let provider = self.get_provider(name)
+            .ok_or_else(|| format!("Unknown Box type: {}", name))?;
+        
+        match provider {
+            BoxProvider::Builtin(constructor) => {
+                // ビルトイン実装を直接呼び出し
+                constructor(args)
+            }
+            BoxProvider::Plugin(plugin_name) => {
+                // TODO: プラグインローダーと連携
+                Err(format!("Plugin loading not yet implemented: {}", plugin_name))
+            }
+        }
+    }
+}
+
+impl Clone for BoxProvider {
+    fn clone(&self) -> Self {
+        match self {
+            BoxProvider::Builtin(f) => BoxProvider::Builtin(*f),
+            BoxProvider::Plugin(name) => BoxProvider::Plugin(name.clone()),
+        }
+    }
+}
+
+// グローバルレジストリインスタンス
+use once_cell::sync::Lazy;
+
+static GLOBAL_REGISTRY: Lazy<Arc<BoxFactoryRegistry>> = 
+    Lazy::new(|| Arc::new(BoxFactoryRegistry::new()));
+
+/// グローバルレジストリを取得
+pub fn get_global_registry() -> Arc<BoxFactoryRegistry> {
+    GLOBAL_REGISTRY.clone()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::box_trait::StringBox;
+    
+    fn test_string_constructor(args: &[Box<dyn NyashBox>]) -> Result<Box<dyn NyashBox>, String> {
+        if args.is_empty() {
+            Ok(Box::new(StringBox::new("")))
+        } else {
+            Ok(Box::new(StringBox::new(&args[0].to_string_box().value)))
+        }
+    }
+    
+    #[test]
+    fn test_builtin_registration() {
+        let registry = BoxFactoryRegistry::new();
+        registry.register_builtin("StringBox", test_string_constructor);
+        
+        let result = registry.create_box("StringBox", &[]).unwrap();
+        assert_eq!(result.to_string_box().value, "");
+    }
+    
+    #[test]
+    fn test_plugin_override() {
+        let registry = BoxFactoryRegistry::new();
+        registry.register_builtin("FileBox", test_string_constructor);
+        
+        // プラグイン設定で上書き
+        let mut config = PluginConfig::default();
+        config.plugins.insert("FileBox".to_string(), "filebox".to_string());
+        registry.apply_plugin_config(&config);
+        
+        // プロバイダーがプラグインに変わっているか確認
+        match registry.get_provider("FileBox").unwrap() {
+            BoxProvider::Plugin(name) => assert_eq!(name, "filebox"),
+            _ => panic!("Expected plugin provider"),
+        }
+    }
+}