feat(9.75f-1): Implement plugin loader and interpreter integration

- Add plugin_loader.rs with FileBoxProxy implementation
- Integrate dynamic FileBox into interpreter (execute_new, method calls)
- Add feature flag 'dynamic-file' support throughout
- Create test program test_dynamic_filebox.nyash
- Plugin builds in 2.86s (vs main build 2+ minutes\!)

Build time improvement confirmed:
- Plugin-only build: 2.86s ✨
- Main build: 2+ minutes (timeout)

Next: Complete testing once main build finishes

src/interpreter/mod.rs

@@ -40,10 +40,16 @@ mod math_methods;
 mod system_methods;
 mod web_methods;
 mod special_methods;
+#[cfg(feature = "dynamic-file")]
+mod plugin_loader;
 
 // Main interpreter implementation - will be moved from interpreter.rs
 pub use core::NyashInterpreter;
 
+// Dynamic plugin support
+#[cfg(feature = "dynamic-file")]
+pub use plugin_loader::{PluginLoader, FileBoxProxy};
+
 
 /// 実行制御フロー
 #[derive(Debug)]