feat: Windows GUI development with egui - simple notepad example

- Fixed EguiBox Send+Sync trait bounds for thread safety
- Added missing Arc import for EguiBox implementation
- Simplified font setup in Windows notepad example
- Successfully built 4.5MB Windows GUI executable
- Nyash can now create native Windows desktop applications! 🎉

src/interpreter/expressions/calls.rs

@@ -734,7 +734,7 @@ impl NyashInterpreter {
         // 🔥 Phase 8.8: pack透明化システム - ビルトインBox判定
         use crate::box_trait::is_builtin_box;
         
-        let is_builtin = is_builtin_box(parent);
+        let mut is_builtin = is_builtin_box(parent);
         
         // GUI機能が有効な場合はEguiBoxも追加判定
         #[cfg(all(feature = "gui", not(target_arch = "wasm32")))]