Fix animation stagger delay on SPA navigation

On View Transitions navigation, elements in the viewport now appear
instantly instead of getting staggered delays that caused the hero
image to show before text. Below-the-fold elements still animate
with stagger on scroll. Also disconnects old IntersectionObserver
before creating a new one to prevent memory leaks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>