Modules and visibility

Rust modules are not files by accident; they are the visibility and namespace system. This theme covers mod, use, pub, pub(crate), and paths with crate:: and super::. Planned exercises: 1. choose mod syntax. 2. recognise private-by-default items. 3. choose pub for exported APIs. 4. fill a use path. 5. fill pub(crate). 6. write a small module API. 7. write imports from sibling modules. 8. translate TS exports/imports. 9. fix a private-field or private-item compile error.