Function build_forest 

pub fn build_forest(items: &[BacklogItem]) -> Forest

Classify items into a Forest by parent membership within the same set.

An item is a root when it has no parent, its parent is not part of items (e.g. filtered out or living in another column), or it points at itself. Items reachable only through a parent-link cycle land in Forest::unreachable so nothing is dropped from the view.