Function trace_impact_closure 

pub fn trace_impact_closure(
    graph: &ModuleGraph,
    root: &Path,
    file_path: &str,
) -> Option<ImpactClosureTrace>

Compute the impact closure for a single file as the seed.

Resolves file_path to a graph FileId, walks reverse_deps + re-export chains to the transitive affected set, and reports the coordination gap (the seed’s exported contracts consumed by modules outside the seed). Returns None when the file is not in the module graph.