Site v0.2.0 — Loop hardening

Branded 404 page; favicon + Apple touch icon + theme-color; OpenGraph PNG image (1200x630, build-time static); Cloudflare Pages _headers (X-Content-Type-Options, X-Frame-Options, Referrer-Policy, Permissions-Policy, HSTS 2-year preload); skip-to-content link + visible focus rings + reduced-motion respect (WCAG 2.1 AA); anchor-linkable H2 headings; DEPLOY.md operator runbook; SECURITY.md + RFC 9116 .well-known/security.txt; scripts/verify-live.sh post-deploy sanity check.