fix: add stable root favicon fallback

[ClaudiuSchuster]

Summary

• Add a stable root favicon.ico generated from the existing prismatic SVG in
  16, 32, 48 and 64 pixel variants.
• Prefer the stable ICO in the page head while retaining the SVG and 64px PNG
  fallbacks.
• Document and validate the ICO dimensions alongside the existing static-site
  checks.

Before this change the live GitHub Pages site served favicon.svg and
favicon.png, but /favicon.ico returned 404, which is the conventional
domain-level lookup used by many crawlers and favicon services.

Verification

• git diff --check
• xmllint --noout favicon.svg
• ICO contains 16x16, 32x32, 48x48 and 64x64 images
• Local HTTP preview serves index, ICO and SVG with byte-identical payloads
• Local HTML reference audit passes with no duplicate IDs or missing local files
• Browser preview checked in the in-app browser; no layout changes were made

Safety

• No external scripts, tracking, cookies or runtime dependencies added.
• No changes to repository settings or GitHub Pages configuration.