inspect-desktop-application reported a regular-file, missing or read-denied
root as artifact_operation_failed/execution_failure, leaving the ENOTDIR,
ENOENT or EACCES cause only in message text.
DirectoryArtifactReader now marks root admission and root-open failures with
DirectoryArtifactRootFailure. DesktopApplicationReader, which owns the caller's
selection, maps a non-directory root to unsupported_target and host failures to
the shared BinaryTargetError (target_unavailable, or access_denied with
read-access recovery), keeping any partial observation. Child-entry failures
and other DirectoryArtifactReader consumers are unchanged.
Fixes#5513
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>