Add GH Pages landing page mock under site/

Single self-contained HTML file at site/index.html with the
terminal/dark aesthetic adapted from the Yinger reference: Basalt
Black / Quartz White palette, IBM Plex Mono / Space Grotesk /
JetBrains Mono via Google Fonts, pill ghost buttons, 64px section
rhythm. No build step or JS deps beyond one rail-hover handler.

Sections: hero with CSS-only stylised inspector preview, problem
statement, catalog stats grid annotated by wired / partial /
pending status (linking #17 #18 #19 #20 #21 for catalogs without
UI consumers yet), 7-row precedence diagram, app status with run
instructions, deep-dives cards, contribute, footer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: samkeen