ONE file, one spine

ONE file, one spine

The engine is a SINGLE file: static/system.css, edited in-repo (the old four-file split and the "canonical source lives elsewhere / read-mostly" doctrine are both retired). Concern boundaries survive as @layer sections inside the file, opened by one spine statement:

@layer reset, core.color, core.type, core.layout, theme, base,
       components, utility, media, skin, view-transitions;

Layers, not source order, decide the cascade — read the whole file before changing it (the layer order IS the cascade; a rule's position in the file tells you nothing about who wins).