findParentCached function
O(1) per step parent lookup using the document's cached parent map.
Returns the direct parent FNode of node, or null if node is the root.
Implementation
FNode? findParentCached(FluentDocument document, FNode node) {
final parentId = document.findParentCached(node.id);
if (parentId == null) return null;
return document.nodeById(parentId);
}