✅ Phase 3 complete: DateTimeBox fully implemented and tested

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

src/interpreter/system_methods.rs

@@ -11,6 +11,7 @@
 
 use super::*;
 use crate::box_trait::StringBox;
+use crate::boxes::{TimeBox, DateTimeBox};
 
 impl NyashInterpreter {
     /// TimeBoxのメソッド呼び出しを実行
@@ -171,6 +172,14 @@ impl NyashInterpreter {
                 }
                 Ok(datetime_box.addHours(arg_values[0].clone_box()))
             }
+            "toString" => {
+                if !arg_values.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("toString() expects 0 arguments, got {}", arg_values.len()),
+                    });
+                }
+                Ok(Box::new(datetime_box.to_string_box()))
+            }
             _ => {
                 Err(RuntimeError::InvalidOperation {
                     message: format!("Unknown DateTimeBox method: {}", method),