The ghost-prune loop in reconcileUnifiedDesktopHalves used existsSync on
the marker's source, which answers false for EACCES/EPERM as well as
ENOENT, so a mode-000 / ACL-denied package folder was treated as an
uninstall and its materialized half rm -rf'd with no warning.
Distinguish a genuinely missing source (ENOENT/ENOTDIR) from one the app
is not allowed to stat: keep the half and warn. materializeDesktopHalf
now also warns on a non-ENOENT stat failure instead of swallowing it.
Part of #111804
b74f158b0d