Function partner_index 

pub fn partner_index(rows: &[CouplingRow]) -> HashMap<String, HashSet<String>>

Build a bidirectional partner index from coupling rows: for each file, the set of files it is Fisher-significantly coupled with. The structure×history fusion analyses (unstable-interface, crossing) intersect this with the import graph, so the construction lives here next to CouplingRow rather than being re-derived in each.