Function build 

pub fn build(
    tasks: &BTreeMap<String, Task>,
    deps: &BTreeMap<String, Dependency>,
    statuses: &BTreeMap<String, ResolvedStatus>,
) -> IssueGraph

Build the graph. statuses maps task id -> resolved status (any precision; the graph only reads the open/closed bit). Tasks referenced by an edge but absent from tasks become stub nodes rather than dropped edges.