cleanup: Remove temporary file ExternCall test code

- Remove file static box from stdlib/mod.rs
- Remove file.read/write/exists ExternCall handling from MIR builder
- Delete test_file_ffi.nyash test file
- Clean build successful

src/stdlib/mod.rs

@@ -65,9 +65,6 @@ impl BuiltinStdlib {
         // console static box
         nyashstd.static_boxes.insert("console".to_string(), Self::create_console_box());
         
-        // file static box (FFI-ABI demonstration)
-        nyashstd.static_boxes.insert("file".to_string(), Self::create_file_box());
-        
         self.namespaces.insert("nyashstd".to_string(), nyashstd);
     }
     
@@ -225,89 +222,4 @@ impl BuiltinStdlib {
         
         console_box
     }
-    
-    /// file static boxを作成 (FFI-ABI demonstration)
-    fn create_file_box() -> BuiltinStaticBox {
-        let mut file_box = BuiltinStaticBox {
-            name: "file".to_string(),
-            methods: HashMap::new(),
-        };
-        
-        // file.read(path) -> string (or null on error)
-        file_box.methods.insert("read".to_string(), |args| {
-            if args.len() != 1 {
-                return Err(RuntimeError::InvalidOperation {
-                    message: "file.read() takes exactly 1 argument".to_string()
-                });
-            }
-            
-            // StringBoxにダウンキャスト
-            if let Some(path_arg) = args[0].as_any().downcast_ref::<StringBox>() {
-                // Rust標準ライブラリでファイル読み込み
-                match std::fs::read_to_string(&path_arg.value) {
-                    Ok(content) => Ok(Box::new(StringBox::new(content))),
-                    Err(_) => {
-                        // エラー時はnullを返す
-                        use crate::boxes::NullBox;
-                        Ok(Box::new(NullBox::new()))
-                    }
-                }
-            } else {
-                Err(RuntimeError::TypeError {
-                    message: format!("file.read() expects string argument, got {:?}", args[0].type_name())
-                })
-            }
-        });
-        
-        // file.write(path, content) -> void
-        file_box.methods.insert("write".to_string(), |args| {
-            if args.len() != 2 {
-                return Err(RuntimeError::InvalidOperation {
-                    message: "file.write() takes exactly 2 arguments".to_string()
-                });
-            }
-            
-            // 両方の引数をStringBoxにダウンキャスト
-            if let (Some(path_arg), Some(content_arg)) = 
-                (args[0].as_any().downcast_ref::<StringBox>(), 
-                 args[1].as_any().downcast_ref::<StringBox>()) {
-                // Rust標準ライブラリでファイル書き込み
-                if let Err(e) = std::fs::write(&path_arg.value, &content_arg.value) {
-                    return Err(RuntimeError::InvalidOperation {
-                        message: format!("Failed to write file: {}", e)
-                    });
-                }
-                
-                // VoidBoxを返す
-                use crate::box_trait::VoidBox;
-                Ok(Box::new(VoidBox::new()))
-            } else {
-                Err(RuntimeError::TypeError {
-                    message: "file.write() expects two string arguments".to_string()
-                })
-            }
-        });
-        
-        // file.exists(path) -> bool
-        file_box.methods.insert("exists".to_string(), |args| {
-            if args.len() != 1 {
-                return Err(RuntimeError::InvalidOperation {
-                    message: "file.exists() takes exactly 1 argument".to_string()
-                });
-            }
-            
-            // StringBoxにダウンキャスト
-            if let Some(path_arg) = args[0].as_any().downcast_ref::<StringBox>() {
-                // Rust標準ライブラリでファイル存在確認
-                let exists = std::path::Path::new(&path_arg.value).exists();
-                Ok(Box::new(BoolBox::new(exists)))
-            } else {
-                Err(RuntimeError::TypeError {
-                    message: format!("file.exists() expects string argument, got {:?}", args[0].type_name())
-                })
-            }
-        });
-        
-        file_box
-    }
 }