fix: Fix NyashParser import path and improve BoxCall fallback

- Fix NyashParser import path in vm.rs tests
- Improve BoxCall fallback logic for plugin/builtin methods
- Add proper function existence checks before lowering to Call

This ensures plugin Box methods correctly fall back to BoxCall
when the corresponding user-defined function doesn't exist.

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

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

src/backend/vm.rs

@@ -1072,7 +1072,7 @@ impl Default for VM {
 mod tests {
     use super::*;
     use crate::mir::{MirModule, MirFunction, FunctionSignature, MirType, EffectMask, BasicBlock};
-    use crate::NyashParser;
+    use crate::parser::NyashParser;
     use crate::runtime::NyashRuntime;
     use crate::core::model::BoxDeclaration as CoreBoxDecl;
     use crate::interpreter::SharedState;