refactor: Material designator with less boilerplate (#5221)

1. **Replace virtual dispatch with `std::variant`** – `DesignatorBase`
polymorphism is replaced by a `std::variant`-based design.
2. **Unify `ProtoDesignator`** – Single template for cylinder, cuboid,
trapezoid, and diamond bounds.
3. **Use `boost::core::demangle`** – Remove `shellTypeName()` and use
`boost::core::demangle` for error messages.

Author: paulgessinger