New density guard at 390x844 and 360x780 on the dashboard and
/insights: page scrollWidth never exceeds the viewport, and no
descendant's visible box crosses its card / hero / list-row edge
(clip-aware below the tile; content cut off at the tile's own edge
counts as an escape). The snapshot mock gains hero fixtures — layout
heroVisible, three score rings, and a briefing whose signal rows pair
long wrapping headlines with wide delta strings — so the guard
exercises the exact shape that regressed, and asserts the spotlight
rows actually render rather than passing vacuously.
