🔧 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/result/mod.rs

@@ -38,6 +38,12 @@ impl NyashResultBox {
 
 impl NyashBox for NyashResultBox {
     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()
+    }
+
         match self {
             NyashResultBox::Ok(val) => Box::new(NyashResultBox::Ok(val.clone_box())),
             NyashResultBox::Err(err) => Box::new(NyashResultBox::Err(err.clone_box())),