Facet

The concern grouping a product's UNIVERSAL cross-kind merchandising facets — the chosen, filterable token attributes every product can be identified and differentiated by regardless of kind: color, material, size (and the size system it is expressed in), pattern, and the like, declared on root `Product`. Distinct from `measurement` (a kind's spec sheet — quantitative extents plus kind-specific categorical settings) and from the storefront-treatment concerns (pricing, availability, fulfilment, …). Whether a facet is a BUYABLE variation axis is NOT a fact of this grouping: it is the per-kind `axis` mark, judged once per kind (color is one held value on a solid tee yet the splitting axis on a tie-dye). This concern re-founds the retired `variation` concern, whose sole job — naming 'the selectable axes' — is now carried by that mark.