Joins Tailwind class lists so that a later class replaces an earlier one of the same kind.

Every component builds its classes as [base, variant, color, size, @class] and passes the list here, so <.button class="rounded-full px-8"> drops the default rounded-lg px-5 instead of leaving both in the attribute and letting the stylesheet order decide.

A class is reduced to a key made of its variants (dark:hover:), its important flag and its group (bg-color, px, rounded-tl). A later class removes every earlier class whose key is the same or belongs to a group it covers (p-4 removes px-2 and pt-1). Classes that belong to no known group are always kept, in order.

Results are cached in ETS, since the same lists are merged on every render.