feat: Phase 9.78e complete - instance_v2 migration with legacy compatibility

- instance_v2 now includes legacy compatibility layer
- All interpreter code migrated to use instance_v2
- Added legacy field access methods (get_fields, set_field_legacy, etc.)
- Fixed type conversion issues (NyashValue vs SharedNyashBox)
- instance.rs still exists but no longer used in interpreter
- TODO: Remove instance.rs completely in next phase
- TODO: Implement proper SharedNyashBox -> NyashValue conversion

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

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

local_tests/test_stringbox_unified.nyash

@@ -0,0 +1,16 @@
+// 🎯 StringBox統一テスト
+
+print("=== StringBox統一テスト ===")
+
+// StringBox作成
+local str = new StringBox("Hello World")
+
+// 統一確認
+print("type_name: " + str.type_name())
+print("content: " + str.toString()) 
+
+// フィールドアクセステスト（統一後の機能）
+str.custom_field = "Added Field"
+print("custom_field: " + str.custom_field)
+
+print("StringBox統一テスト完了")