Remove CEF: the web content crate, ElyMark's page, their packaging, licences and tests (C14.5)

PRD and agents.md name the native route; ElyMail's body will not use
CEF or Electron. ElyMark's package carries the KaTeX fonts' OFL.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/engines-key.sh

@@ -4,5 +4,5 @@ set -euo pipefail
 root=$(cd "$(dirname "$0")/.." && pwd)
 cd "$root"
 find scripts/fetch-references.sh scripts/apply-lo-patches.sh scripts/build-libreoffice.sh scripts/build-mupdf.sh \
-  scripts/build-git.sh scripts/fetch-git-lfs.sh scripts/fetch-tessdata.sh scripts/fetch-cef.sh \
+  scripts/build-git.sh scripts/fetch-git-lfs.sh scripts/fetch-tessdata.sh \
   packaging/libreoffice patches -type f | LC_ALL=C sort | xargs shasum -a 256 | shasum -a 256 | cut -c1-12