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/bid/generic_plugin_box.rs

@@ -2,8 +2,6 @@
 mod plugin_impl {
 
 use crate::bid::{BidError, BidResult, LoadedPlugin};
-use crate::bid::tlv::{TlvEncoder, TlvDecoder};
-use crate::bid::types::BidTag;
 use crate::box_trait::{NyashBox, StringBox, BoolBox, BoxCore, BoxBase};
 use std::any::Any;
 use std::fmt;