Interior mutability

Sometimes Rust permits mutation through a shared reference by moving borrow checks to runtime. This theme teaches Cell, RefCell, and the Rc<RefCell<T>> pattern as tools with costs, not escape hatches. Planned exercises: 1. choose RefCell for runtime-checked mutation. 2. recognise borrow_mut. 3. choose Cell for small copy values. 4. fill RefCell::new. 5. fill a scoped mutable borrow. 6. write a counter using Cell. 7. write a shared mutable graph edge with Rc<RefCell<_>>. 8. translate TS shared mutation deliberately. 9. fix a runtime borrow panic by shortening a borrow.