Creates useNearViewport hook (IntersectionObserver, 200px rootMargin,
one-shot disconnect after first trigger). ImageContent and VideoContent
now gate loadSrc() on nearViewport — when autoPlay is enabled, encrypted
media is not decrypted until the element is within 200px of the visible
area, reducing initial page load cost on long timelines.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
