Published artifacts handle a third theme state

The publishing prompt and the artifact-design guidance now treat the viewer as having three theme states rather than two: explicitly dark, explicitly light, and an unstamped default where only the browser's prefers-color-scheme decides. Generated pages are told to define a full light palette on :root, guard the dark media block so an explicit light choice wins, add a dark override, and set an explicit background on body so a transparent page does not pick up the host's colour.