Arc and Mutex: shared state across threads

`Arc<T>` is `Rc<T>` with an atomic count, so it can cross thread boundaries. `Mutex<T>` guards the data inside it: you cannot read or write without locking first, which makes the lock impossible to forget.

`lock()` returns a guard. The lock is released when the guard is dropped, so scoping matters — hold it for as little as possible.

The `Send` and `Sync` marker traits are what the compiler checks here. A type that is not safe to share simply will not compile into this shape, which is where 'fearless concurrency' comes from.

Run it and change it

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