Add btravstack landing page

Static landing page for the btravstack TypeScript-backend libraries,
compiled from the Claude Design "btravstack" .dc.html into plain
HTML/CSS for GitHub Pages (no build step). Includes favicons, an OG
social card, and click-to-copy install commands.

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

Author: btravers