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,
 }