fix: Replace static mut with Lazy for thread safety in FileBoxRegistry

- Eliminate static_mut_refs warnings by using once_cell::sync::Lazy
- Make FileMode enum public to fix private_interfaces warning
- Reduce warnings from 6 to 3
- Prepare for Rust 2024 edition compatibility

src/interpreter/core.rs

@@ -225,6 +225,7 @@ pub struct NyashInterpreter {
     pub(super) stdlib: Option<BuiltinStdlib>,
 
     /// 共有ランタイム（Boxレジストリ等）
+    #[allow(dead_code)]
     pub(super) runtime: NyashRuntime,
 }
 

src/interpreter/objects.rs

@@ -1127,7 +1127,7 @@ impl NyashInterpreter {
             // 🔥 Phase 8.8: pack透明化システム - ビルトインBox判定
             use crate::box_trait::is_builtin_box;
             
-            let mut is_builtin = is_builtin_box(parent_name);
+            let is_builtin = is_builtin_box(parent_name);
             
             // GUI機能が有効な場合はEguiBoxも追加判定
             #[cfg(all(feature = "gui", not(target_arch = "wasm32")))]