Open with two pinned scroll scenes

Scene one holds the viewport while the headline gives way to the
running app; scene two pins the app and walks three callouts past it.
Scroll drives every transform directly, so the reader scrubs the film
and can play it backwards. Small screens and reduced motion get the
same content as a static column — pinned scenes are jank on phones.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>