feat: add IntersectionObserver staggered fade-in to ValuePropositionSection (Closes #1267)

[waterWang]

Summary

Adds IntersectionObserver-driven staggered fade-in animation to the four feature cards in ValuePropositionSection.tsx. Each card animates independently with an 80ms incremental delay (0, 80, 160, 240ms) so the reveal reads left-to-right/top-to-bottom. The animation fires once per page load when the section scrolls into view.

Changes

• src/components/ValuePropositionSection.tsx: Added useIntersectionOnce hook using IntersectionObserver; headline/subhead/cards now fade in from translateY(12px/20px) to translateY(0) using var(--transition-base) (200ms ease-in-out); each card has staggered transitionDelay.
• src/components/tests/ValuePropositionSection.test.tsx: Mock IntersectionObserver; tests for start-hidden, reveal-on-intersection, and one-shot disconnect behavior.

Closes #1267