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/expressions/calls.rs

@@ -255,6 +255,14 @@ impl NyashInterpreter {
             return self.execute_file_method(file_box, method, arguments);
         }
         
+        // FileBoxProxy method calls (動的ライブラリ版)
+        #[cfg(feature = "dynamic-file")]
+        {
+            if let Some(file_proxy) = obj_value.as_any().downcast_ref::<crate::interpreter::plugin_loader::FileBoxProxy>() {
+                return self.execute_file_proxy_method(file_proxy, method, arguments);
+            }
+        }
+        
         // ResultBox method calls
         if let Some(result_box) = obj_value.as_any().downcast_ref::<crate::box_trait::ResultBox>() {
             return self.execute_result_method(result_box, method, arguments);