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>

src/method_box.rs

@@ -7,7 +7,7 @@
 
 use crate::box_trait::{NyashBox, StringBox, BoolBox, BoxCore, BoxBase};
 use crate::ast::ASTNode;
-use crate::instance::InstanceBox;
+use crate::instance_v2::InstanceBox;
 use std::fmt::{Debug, Display};
 use std::any::Any;
 use std::sync::{Arc, Mutex};