Class-string composition for LanternUI components.

merge/1 flattens nested class lists, drops falsy fragments, and joins — so conditional class lists from HEEx compose cleanly with a component's base classes:

LanternUI.Class.merge(["lui-btn", @wide && "lui-btn-wide", @class])

No Tailwind-conflict engine is needed by design: LanternUI's own classes are namespaced semantic classes (lui-*), never Tailwind utilities, so a consumer's class= override cannot conflict with a component base. (We evaluated twix/tails/tailwind_merge; twix crashes on non-Tailwind class names via binary_to_existing_atom, and none is needed for this model — dropping the dependency also keeps the package fully self-contained.)

variant/3 selects a class fragment from a variants map.