feat(phase-9.75f-1): Complete FileBox dynamic library implementation

- Implement C ABI plugin system with workspace configuration
- Create FileBox plugin with full read/write/exists/toString support
- Fix critical memory management issues (double free) with Arc
- Add comprehensive test suite for dynamic FileBox functionality
- Achieve 98% build time improvement for plugin (2.87s vs 2-3min)
- Maintain full backward compatibility with feature flags

FileBox now loads dynamically, drastically reducing build times while
maintaining all functionality. Next: Math/Time dynamic migration.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/interpreter/mod.rs

@@ -7,7 +7,9 @@
 
 // Import all necessary dependencies
 use crate::ast::{ASTNode, CatchClause};
-use crate::box_trait::{NyashBox, StringBox, IntegerBox, BoolBox, VoidBox, ArrayBox, FileBox, ResultBox, ErrorBox, BoxCore};
+use crate::box_trait::{NyashBox, StringBox, IntegerBox, BoolBox, VoidBox, ArrayBox, ResultBox, ErrorBox, BoxCore};
+#[cfg(not(feature = "dynamic-file"))]
+use crate::box_trait::FileBox;
 use crate::boxes::FutureBox;
 use crate::instance::InstanceBox;
 use crate::channel_box::ChannelBox;