redesign: da Vinci × Apple showcase site with interactive crayon brain (#18)

Full rebuild of docs-site/ — Apple-clean layout (sticky translucent nav,
scroll-reveal, premium type rhythm) with a Leonardo da Vinci anatomical-
manuscript skin (parchment, ink linework, Cormorant/EB Garamond + Caveat,
vitruvian guides, mirror-writing footer) and a hand-drawn crayon brain.

- Interactive SVG brain (rough.js, vendored locally — no CDN SPOF): regions
  reflect coverage status; neurons fire pulses on hover/focus/click; data-driven
  from memory_coverage.json; keyboard accessible; reduced-motion honored.
- Coverage ring (~60%), CLS folding-loop diagram, 9 real-number benchmark cards
  (no fabricated competitor numbers; LongMemEval omitted pending a confirmed figure).
- Social preview (OG + Twitter + og-image.svg), favicon, font fallbacks, and
  three-layer graceful degradation (never a blank section).


Claude-Session: https://claude.ai/code/session_018Cafgnpw3v7U3f2Lt1ASxs

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: duanyiqun