Function get_callers 

pub fn get_callers(
    project: &ProjectRoot,
    function_name: &str,
    file_path: Option<&str>,
    max_results: usize,
    graph_cache: Option<&GraphCache>,
) -> Result<Vec<CallerEntry>>

Find all functions that call function_name across the project. Edges are resolved via the 6-stage confidence cascade when an import graph is available.