Windows and Linux build environments; check clean on both (E-01)

MuPDF builds with MSBuild on Windows (/MT, OpenSSL from vcpkg);
Git for Windows' MinGit stands in for the source build there. CEF
loads from where libcef lives and sets its API version first on
every platform. File identity on Windows compares volume and index.
The checkout and reference/ keep LF on Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/fetch-cef.sh

@@ -4,7 +4,9 @@ set -euo pipefail
 
 root=$(cd "$(dirname "$0")/.." && pwd)
 [ -d "$root/reference/cef-rs" ] || { echo "fetch-cef: run scripts/fetch-references.sh first" >&2; exit 1; }
-work=$(mktemp -d)
+# Not the system temp: Windows may block programs there.
+mkdir -p "$root/engines"
+work=$(mktemp -d "$root/engines/cef-export.XXXXXX")
 trap 'rm -rf "$work"' EXIT
 cd "$root/reference/cef-rs"
 # The exporter checks each archive against CEF's own index.