Function process_files_parallel 

pub fn process_files_parallel<T, F>(
    files: Vec<String>,
    num_workers: usize,
    processor: F,
) -> Vec<T>
where
    T: Send + 'static,
    F: Fn(String) -> T + Send + Sync + 'static,

Process files in parallel with a worker pool.

Distributes file processing across multiple threads for faster indexing.