Add root ICO favicon fallback

[ClaudiuSchuster]

Summary

• add a root favicon.ico derived from the existing OSS Singularity brand mark
• include the checked-in icon in the deterministic site build and static release allowlists
• keep the existing SVG favicon, sitemap, robots policy, and agent data unchanged

Verification

• git diff --check
• ./scripts/check-repository.sh
• Node.js 24 Commons and browser-controller suites
• complete Python release-artifact, Commons-artifact, rehearsal, candidate, required-check, transition, remote, and publication suites
• deterministic seven-size ICO inspection with file and icotool
• source-to-dist byte comparison for favicon.ico
• site-boundary check including reachable history and GitHub collaboration text

The ICO is a static derivative of the existing local vector asset; no runtime dependency or third-party asset is added. The sitemap, robots.txt, agent data, Commons service, and server policy remain outside the change.