feat: Implement Phase 9.78e instance_v2 migration with unified registry

Major achievements:
- ✅ UserDefinedBoxFactory implementation with unified registry integration
- ✅ Constructor execution for user-defined boxes (Person init working)
- ✅ Import path fixes across interpreter modules
- ✅ unwrap_instance helper function for InstanceBox operator support

Technical details:
- Modified UnifiedBoxRegistry to handle empty box_types() factories
- Implemented constructor execution in execute_new for InstanceBox
- Added unwrap_instance helper to handle InstanceBox wrapping in operators
- Updated CURRENT_TASK.md with detailed progress tracking

Next: Fix 4 operator functions to complete InstanceBox operator support

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

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

src/interpreter/special_methods.rs

@@ -15,6 +15,7 @@
 use super::*;
 use crate::boxes::SoundBox;
 use crate::method_box::MethodBox;
+use crate::instance_v2::InstanceBox;
 
 impl NyashInterpreter {
     /// SoundBoxのメソッド呼び出しを実行
@@ -159,7 +160,7 @@ impl NyashInterpreter {
         let instance = instance_arc.lock().unwrap();
         
         // InstanceBoxにダウンキャスト
-        if let Some(instance_box) = instance.as_any().downcast_ref::<crate::instance_v2::InstanceBox>() {
+        if let Some(instance_box) = instance.as_any().downcast_ref::<InstanceBox>() {
             // メソッドを取得
             let method_ast = instance_box.get_method(&method_box.method_name)
                 .ok_or(RuntimeError::InvalidOperation {