feat(9.75f-1): Implement plugin loader and interpreter integration

- Add plugin_loader.rs with FileBoxProxy implementation
- Integrate dynamic FileBox into interpreter (execute_new, method calls)
- Add feature flag 'dynamic-file' support throughout
- Create test program test_dynamic_filebox.nyash
- Plugin builds in 2.86s (vs main build 2+ minutes\!)

Build time improvement confirmed:
- Plugin-only build: 2.86s ✨
- Main build: 2+ minutes (timeout)

Next: Complete testing once main build finishes

src/interpreter/objects.rs

@@ -96,9 +96,20 @@ impl NyashInterpreter {
                 }
                 let path_value = self.execute_expression(&arguments[0])?;
                 if let Some(path_str) = path_value.as_any().downcast_ref::<StringBox>() {
-                    let file_box = Box::new(FileBox::new(&path_str.value)) as Box<dyn NyashBox>;
-                    // 🌍 革命的実装：Environment tracking廃止
-                    return Ok(file_box);
+                    #[cfg(feature = "dynamic-file")]
+                    {
+                        // 動的ライブラリ経由でFileBoxを作成
+                        use crate::interpreter::plugin_loader::PluginLoader;
+                        let file_box = PluginLoader::create_file_box(&path_str.value)?;
+                        return Ok(file_box);
+                    }
+                    
+                    #[cfg(not(feature = "dynamic-file"))]
+                    {
+                        // 静的リンク版
+                        let file_box = Box::new(FileBox::new(&path_str.value)) as Box<dyn NyashBox>;
+                        return Ok(file_box);
+                    }
                 } else {
                     return Err(RuntimeError::TypeError {
                         message: "FileBox constructor requires string path argument".to_string(),