Mutable borrowing

The borrow checker's headline rule is simple but unfamiliar: many shared borrows or one mutable borrow, never both at the same time. This theme teaches &mut T, reborrowing, and scope as the tool for ending borrows. Planned exercises: 1. choose &mut for an updater. 2. recognise the one-mutable-borrow rule. 3. choose a smaller scope. 4. fill a mutable parameter. 5. fill a mutable call site. 6. write an in-place updater. 7. write code that reads after a mutable borrow ends. 8. translate TS mutation into Rust borrowing. 9. fix overlapping borrow diagnostics.