🔧 Phase A: Add share_box() method to NyashBox trait and all Box implementations

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

src/boxes/file/mod.rs

@@ -134,6 +134,12 @@ impl BoxCore for FileBox {
 
 impl NyashBox for FileBox {
     fn clone_box(&self) -> Box<dyn NyashBox> {
+    
+    /// 仮実装: clone_boxと同じ（後で修正）
+    fn share_box(&self) -> Box<dyn NyashBox> {
+        self.clone_box()
+    }
+
         // Note: Cannot truly clone a File handle, so create a new one to the same path
         match FileBox::open(&self.path) {
             Ok(new_file) => Box::new(new_file),