refactor: implement iterative top-down traversal

bigframes/core/bigframe_node.py

@@ -330,12 +330,32 @@ def top_down(
         """
         Perform a top-down transformation of the BigFrameNode tree.
         """
+        results: Dict[BigFrameNode, BigFrameNode] = {}
+        # Each stack entry is (node, t_node). t_node is None until transform(node) is called.
+        stack: list[tuple[BigFrameNode, typing.Optional[BigFrameNode]]] = [(self, None)]
 
-        @functools.cache
-        def recursive_transform(node: BigFrameNode) -> BigFrameNode:
-            return transform(node).transform_children(recursive_transform)
+        while stack:
+            node, t_node = stack[-1]
+
+            if t_node is None:
+                if node in results:
+                    stack.pop()
+                    continue
+                t_node = transform(node)
+                stack[-1] = (node, t_node)
+
+            all_done = True
+            for child in reversed(t_node.child_nodes):
+                if child not in results:
+                    stack.append((child, None))
+                    all_done = False
+                    break
+
+            if all_done:
+                results[node] = t_node.transform_children(lambda x: results[x])
+                stack.pop()
 
-        return recursive_transform(self)
+        return results[self]
 
     def bottom_up(
         self: BigFrameNode,