Function build_dependency_graph 

pub fn build_dependency_graph(
    rules: &[LemmaRule],
) -> HashMap<String, HashSet<String>>

Build a dependency graph showing which rules depend on which other rules.

Returns a map: rule_name -> set of rule names it depends on. This graph is used for topological sorting to determine execution order and for detecting circular dependencies.

Examples

Given rules:
  rule total = subtotal?
  rule subtotal = price * quantity

Returns:
  {"total": {"subtotal"}, "subtotal": {}}