test: Verify plugin system with feature flags

- Successfully built CounterBox and FileBox plugins in release mode
- All plugin E2E tests passing with --features plugins flag
- CounterBox: 2/2 tests ✅ (basic inc/get, assignment sharing)
- FileBox: 4/4 tests ✅ (close void, delegation, VM, Handle TLV)

プラグインシステムの完全動作確認
- プラグインのリリースビルド成功
- plugins featureフラグでのE2Eテスト全て成功
- 統一レジストリシステムが正常動作

🤖 Generated with Claude Code

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

src/interpreter/core.rs

@@ -547,12 +547,29 @@ impl NyashInterpreter {
     
     /// local変数を宣言（関数内でのみ有効）
     pub(super) fn declare_local_variable(&mut self, name: &str, value: Box<dyn NyashBox>) {
-        self.local_vars.insert(name.to_string(), Arc::from(value));
+        // Pass-by-share for plugin handle types; by-value (clone) semantics can be applied at call sites
+        #[allow(unused_mut)]
+        let mut store_value = value;
+        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+        {
+            if store_value.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                store_value = store_value.share_box();
+            }
+        }
+        self.local_vars.insert(name.to_string(), Arc::from(store_value));
     }
     
     /// outbox変数を宣言（static関数内で所有権移転）
     pub(super) fn declare_outbox_variable(&mut self, name: &str, value: Box<dyn NyashBox>) {
-        self.outbox_vars.insert(name.to_string(), Arc::from(value));
+        #[allow(unused_mut)]
+        let mut store_value = value;
+        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+        {
+            if store_value.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                store_value = store_value.share_box();
+            }
+        }
+        self.outbox_vars.insert(name.to_string(), Arc::from(store_value));
     }
     
     /// local変数スタックを保存・復元（関数呼び出し時）