Module reachability 

Reachability helpers (transitive closure counts).

This module is intentionally small and allocation-light:

• Build adjacency lists once.
• Use a “visited stamp” (Vec<u32>) to avoid re-allocating seen for every start node.

Edges are interpreted as u -> v (directed).

Functions

reachability_counts_edges

Count transitive reachability for each node in a directed graph.