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/box_factory/plugin.rs

@@ -50,7 +50,7 @@ impl BoxFactory for PluginBoxFactory {
     
     fn is_available(&self) -> bool {
         // Check if any plugins are loaded
-        let registry = get_global_registry();
+        let _registry = get_global_registry();
         // TODO: Add method to check if registry has any providers
         true
     }