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/finalization.rs

@@ -11,6 +11,7 @@ use std::collections::HashSet;
 use std::sync::{Arc, Mutex};
 use std::fmt;
 use crate::box_trait::NyashBox;
+use crate::instance_v2::InstanceBox;
 use lazy_static::lazy_static;
 
 lazy_static! {
@@ -67,7 +68,7 @@ impl BoxFinalizer {
             
             if !is_finalized(*box_id) {
                 // fini()メソッドを呼び出す（存在する場合）
-                if let Some(instance) = nyash_box.as_any().downcast_ref::<crate::instance_v2::InstanceBox>() {
+                if let Some(instance) = nyash_box.as_any().downcast_ref::<InstanceBox>() {
                     let _ = instance.fini();
                 }
                 mark_as_finalized(*box_id);