Native form controls over custom ones

A styled `<div>` pretending to be a checkbox has to reimplement keyboard handling, screen reader semantics, and focus behavior: all things the browser's real checkbox already does correctly. Nucleus's forms module styles the native input, select, and radio elements instead, so accessibility isn't a rebuild project on top of the design.

The tradeoff is real: native controls are harder to make look identical across browsers than a custom div is. Nucleus accepts that constraint because the alternative, a control that looks right but behaves wrong for a screen reader user, is the worse failure.