Function relativize_graphs 

pub fn relativize_graphs(
    graphs: &mut PluginGraphs,
    target: &Path,
    roots: &HashMap<String, String>,
)

Rewrite all node path fields to be relative:

• paths under target → plain relative path (src/main.rs)
• paths under a named root → {name}/rest/of/path
• anything else → left as-is