Painting the body's color onto a transparent source html flips CSS 2.1
background propagation: body's background stops propagating to the canvas
and paints above negative-z descendants, burying full-bleed z<0 hero
backdrops under an opaque page background. Emit the html rule only when
the source html actually painted one; fall back to #ffffff only when both
html and body are transparent. Applies to the Next globals, Tailwind, and
multi-route Vite paths via shared helpers.
6 new tests; suite at 322 green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
