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/debug_string_concat.nyash

@@ -0,0 +1,13 @@
+// 🔍 文字列連結デバッグテスト
+
+// 単純な文字列作成
+local str1 = "Hello"
+local str2 = "World"  
+
+print("str1 created")
+print("str2 created")
+
+// 文字列連結テスト（この行でエラーが出るはず）
+local result = str1 + str2
+
+print("Test completed WITH concatenation")