Trait objects and dynamic dispatch

Generics pick one concrete type per call site. When you need a single collection holding several different types that all implement a trait, you need a trait object: `Box<dyn Shape>` or `&dyn Shape`.

That flexibility costs one pointer indirection at call time — the vtable lookup. It is small, and it is the only time Rust adds runtime dispatch without you asking.

`impl Trait` in a return position is the middle ground: the caller gets a single concrete type without you naming it, still statically dispatched.

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