Shared borrowing

Borrowing is how Rust lets code inspect data without taking it. &T references are shared, read-only views whose owner remains somewhere else. Planned exercises: 1. choose &String over String for a read helper. 2. recognise call-site borrowing with &name. 3. choose dereference-free field or method access. 4. fill an & parameter. 5. fill an & argument. 6. write a read-only helper. 7. write a caller that keeps using the owner afterward. 8. translate a TS read helper. 9. fix a function that accidentally takes ownership.