test: green up after Phase 2 merge for #26\n\n- Fix AST tests (is_override)\n- Align MIR builder test with current If AST\n- Adjust ResultBox API usage in tests\n- Add len() helpers to ArrayBox/BufferBox for tests

src/boxes/buffer/mod.rs

@@ -47,6 +47,11 @@ impl BufferBox {
             base: BoxBase::new(),
         }
     }
+
+    /// Rust向けヘルパー: バッファ長をusizeで取得（テスト用）
+    pub fn len(&self) -> usize {
+        self.data.lock().unwrap().len()
+    }
     
     pub fn from_vec(data: Vec<u8>) -> Self {
         BufferBox {