feat(phase-9.75g-0): Implement BID-FFI Day 5 - FileBox plugin library and transparent switching (90% complete)

Day 5 achievements:
- Created independent FileBox plugin crate with C FFI exports
- Integrated plugin loading into Nyash interpreter startup
- Implemented transparent builtin/plugin Box switching via nyash.toml
- Successfully loaded plugin library (385KB .so) at runtime
- Confirmed PluginBox proxy creation for FileBox instances

Architecture changes:
- Added plugins/ directory with .gitignore for build artifacts
- Modified runner.rs to load plugins from nyash.toml on startup
- Updated objects.rs to use BoxFactoryRegistry for FileBox creation
- Fixed bid module visibility between lib.rs and main.rs

Remaining work (10%):
- Complete PluginBox proxy method implementations (toString, etc.)
- Test actual file operations through plugin interface
- Finalize error handling and edge cases

Build status: All tests passing, plugin loading confirmed

plugins/nyash-filebox-plugin/src/filebox.rs

@@ -0,0 +1,173 @@
+//! FileBox実装
+//! 
+//! ファイル操作を提供するBox実装
+
+use std::fs::File;
+use std::io::{Read, Write, Seek, SeekFrom};
+use std::collections::HashMap;
+use std::path::PathBuf;
+
+/// FileBoxインスタンス
+pub struct FileBoxInstance {
+    path: Option<PathBuf>,
+    file: Option<File>,
+    content: String,
+}
+
+impl FileBoxInstance {
+    /// 新しいFileBoxインスタンス作成
+    pub fn new() -> Self {
+        Self {
+            path: None,
+            file: None,
+            content: String::new(),
+        }
+    }
+    
+    /// ファイルオープン
+    pub fn open(&mut self, path: &str) -> Result<(), std::io::Error> {
+        let path_buf = PathBuf::from(path);
+        
+        // ファイルを読み書きモードでオープン
+        let mut file = std::fs::OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path_buf)?;
+        
+        // 既存内容を読み込み
+        let mut content = String::new();
+        if let Err(_) = file.read_to_string(&mut content) {
+            // 読み込めない場合は空文字列
+            content.clear();
+        }
+        
+        // ファイルポインタを先頭に戻す
+        file.seek(SeekFrom::Start(0))?;
+        
+        self.path = Some(path_buf);
+        self.file = Some(file);
+        self.content = content;
+        
+        Ok(())
+    }
+    
+    /// ファイル読み取り
+    pub fn read(&mut self) -> Result<String, std::io::Error> {
+        if let Some(ref mut file) = self.file {
+            let mut content = String::new();
+            file.seek(SeekFrom::Start(0))?;
+            file.read_to_string(&mut content)?;
+            self.content = content.clone();
+            Ok(content)
+        } else {
+            Ok(self.content.clone())
+        }
+    }
+    
+    /// ファイル書き込み
+    pub fn write(&mut self, data: &str) -> Result<(), std::io::Error> {
+        self.content = data.to_string();
+        
+        if let Some(ref mut file) = self.file {
+            file.seek(SeekFrom::Start(0))?;
+            file.set_len(0)?; // ファイル内容をクリア
+            file.write_all(data.as_bytes())?;
+            file.flush()?;
+        }
+        
+        Ok(())
+    }
+    
+    /// ファイルパス取得
+    pub fn path(&self) -> Option<&str> {
+        self.path.as_ref().and_then(|p| p.to_str())
+    }
+    
+    /// 内容取得
+    pub fn content(&self) -> &str {
+        &self.content
+    }
+}
+
+/// FileBoxレジストリ
+pub struct FileBoxRegistry {
+    instances: HashMap<u32, FileBoxInstance>,
+}
+
+impl FileBoxRegistry {
+    pub fn new() -> Self {
+        Self {
+            instances: HashMap::new(),
+        }
+    }
+    
+    pub fn register(&mut self, handle: u32, instance: FileBoxInstance) {
+        self.instances.insert(handle, instance);
+    }
+    
+    pub fn get(&self, handle: u32) -> Option<&FileBoxInstance> {
+        self.instances.get(&handle)
+    }
+    
+    pub fn get_mut(&mut self, handle: u32) -> Option<&mut FileBoxInstance> {
+        self.instances.get_mut(&handle)
+    }
+    
+    pub fn remove(&mut self, handle: u32) -> Option<FileBoxInstance> {
+        self.instances.remove(&handle)
+    }
+    
+    pub fn count(&self) -> usize {
+        self.instances.len()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+    
+    #[test]
+    fn test_filebox_basic_operations() {
+        let mut filebox = FileBoxInstance::new();
+        
+        // テスト用ファイル
+        let test_file = "test_plugin_file.txt";
+        
+        // ファイルオープン
+        assert!(filebox.open(test_file).is_ok());
+        assert_eq!(filebox.path(), Some(test_file));
+        
+        // 書き込み
+        assert!(filebox.write("Hello from plugin!").is_ok());
+        assert_eq!(filebox.content(), "Hello from plugin!");
+        
+        // 読み込み
+        let content = filebox.read().unwrap();
+        assert_eq!(content, "Hello from plugin!");
+        
+        // クリーンアップ
+        let _ = fs::remove_file(test_file);
+    }
+    
+    #[test]
+    fn test_filebox_registry() {
+        let mut registry = FileBoxRegistry::new();
+        
+        let filebox1 = FileBoxInstance::new();
+        let filebox2 = FileBoxInstance::new();
+        
+        registry.register(1, filebox1);
+        registry.register(2, filebox2);
+        
+        assert_eq!(registry.count(), 2);
+        assert!(registry.get(1).is_some());
+        assert!(registry.get(2).is_some());
+        assert!(registry.get(3).is_none());
+        
+        registry.remove(1);
+        assert_eq!(registry.count(), 1);
+        assert!(registry.get(1).is_none());
+    }
+}