feat: useScrollTrigger, useTransform, useEffectEvent

[clementroche]

Summary

• useScrollTrigger — High-performance scroll progress tracking with GSAP ScrollTrigger-like position syntax. Zero per-frame DOM reads, zero React re-renders. Full control over scroll-driven effects for scrollytelling.
• useTransform / TransformProvider — Context-based transform accumulation so child scroll triggers stay accurate when parents are translated programmatically (parallax).
• useEffectEvent — Polyfill for React's experimental useEffectEvent (stable callback ref for React 17+).
• Dedicated /scroll-trigger playground with 9 interactive demos: basic progress, direction, enter/leave events, position combinations, steps, offset, disabled toggle, CSS custom properties, and TransformProvider.
• Build migration from pnpm+tsup to bun+tsdown.

useScrollTrigger features

• Position syntax: "element-position viewport-position" (top, center, bottom, or pixel values)
• Callbacks: onEnter, onLeave, onProgress with progress, direction, isActive, height, steps
• External rect support: share a single useRect across multiple triggers on the same element
• Lenis integration: automatic, with native scroll fallback
• Infinite scroll support via modulo
• TransformProvider compensation for parallax offsets

Test plan

• Run bun run build — verify clean build
• Run bun dev — verify playground loads at /scroll-trigger
• Scroll through all 9 demo sections
• Verify direction indicator updates in Basic Progress
• Verify enter/leave event log shows correct direction arrows
• Verify TransformProvider demo shows parallax compensation
• Test with Lenis provider wrapping the app