Share a counter across threads with Arc>.
Chain iterator adaptors and collect into a Vec.
Use anyhow::Result and the `?` operator for ergonomic errors.
Derive a clap parser for typed CLI args.
Run concurrent async tasks with tokio::spawn and join.
Use std::thread::spawn and join handles for parallel work.
Walk a single directory level with std::fs::read_dir.
Sort a Vec of structs by one field using sort_by_key.
Compile a regex and extract a named capture group.
Compute a SHA256 hex digest with the sha2 crate.