Idioms and tooling

This final theme is the code-review survival kit: rustfmt, Clippy, docs, Result in main, todo! vs unimplemented!, prelude habits, and avoiding needless clone, unwrap, and type annotations. Planned exercises: 1. choose cargo fmt. 2. recognise a Clippy suggestion. 3. choose expect over blind unwrap when panicking is intentional. 4. fill a main() -> Result. 5. fill a doc comment. 6. write a small polished CLI-style main. 7. remove needless clones. 8. translate a TS script into idiomatic Rust structure. 9. refactor a rough program into the idioms taught across the course.