docs-site: landing round 4 — scene scrolltelling per motion.md

- .lp-scenes: hero / quickstart / demo scenes with named view-timelines and
  sticky full-height stages; scenes stack as you scroll, outgoing sheet sinks
  (scale+fade) while the next rides over; hero photo pushes in on exit
- quickstart sheet: kinetic type (Install. / Start. / Codex, rerouted.) with
  staggered clip-path wipes on the scene timeline; 2-line terminal below,
  vertical layout; full 4-line quickstart moved to a full-width bento cell
- demo scrub scene: demo.gif split into 158 webp frames (12fps, 960w); canvas
  plays through with scroll (480dvh travel, IntersectionObserver lazy start,
  checkpoint-first progressive load, rAF scrub); reduced-motion/<48rem keep
  the animated poster
- bento: asymmetric slab (dominant 2-col cell, tall image cell down the right
  rail, full-width terminal base row), per-cell reveals removed
- fixes: scene stages fill the viewport (no blank gaps); brand chips kept

Author: lidge-jun