v0.5.1 — reinstall reuses game dir + Steam-dir guard
Installer-logic fix (binaries byte-identical to v0.5.0):
- --reinstall / --remove now reuse the game dir recorded at install (per variant), so a machine with both a GOG and a Steam install reuses the right one — no need to re-pass --game-dir.
- The standalone installer (install-xwau-linux.sh) refuses a Steam-library path and points to install-xwau-steam.sh, instead of silently adopting a Steam install.
Upgrading from v0.4.x/v0.5.0: Steam users can just run ./install-xwau-steam.sh --reinstall; GOG users pass --game-dir once (it is then remembered).