v3 replaced hero/background.html's persistent full-viewport blur
overlay with hero-scroll-fade.js, which fades the hero band to
nothing after 500px of scroll - reusing the same
layoutBackgroundBlur flag to gate the new behavior, with no way
back to the old look.
Added layoutBackgroundBlurStyle (fade default, classic opt-in)
to restore the pre-v3 persistent blur overlay, reusing the same
background-blur.js still used by hero/thumbAndBackground.html