Schema for values within an attribute ("White", "Oak", "Anthracite").
key is a stable slug unique within its attribute — generated once by
the context, never cast on update. value holds the primary-language
display text; other languages live in data.
is_default is explicit — display order and commercial default are
independent (values may sort alphabetically while Oak is the standard).
A partial unique index allows at most one default per attribute; the
context flips defaults inside a transaction (unset-then-set).