Schema for attributes within an attribute group ("Color", "Trim").
key is a stable slug unique within its group — the durable identity
that translations can never change. It is generated by the context from
the primary-language name at creation and never cast on update.
kind semantics come from the product design: "fixed" = a single
value shown on the product card; "multi" = a list of options, one of
which is chosen downstream (per order line, in the parent app).