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/interpreter/expressions/access.rs

@@ -6,7 +6,7 @@
 use crate::ast::ASTNode;
 use crate::box_trait::{NyashBox, SharedNyashBox};
 use crate::boxes::FutureBox;
-use crate::instance::InstanceBox;
+use crate::instance_v2::InstanceBox;
 use crate::interpreter::core::{NyashInterpreter, RuntimeError};
 use std::sync::Arc;