dws upgrade registered no --dry-run flag and never read the global one,
so --dry-run fell through to runUpgrade and performed a real, irreversible
upgrade (download + replace binary). This contradicts the flag's documented
contract (预览操作内容，不实际执行).
Resolve the target release and platform asset (so a missing build / 'already
latest' is still reported), then render the planned 1-5 steps and return
before any side effect: no backup, no download, no replace. Advertise
--dry-run in the command examples.
Fixes#364
