XFCE on SDDM; menu defaults to none for DE and TWM

- XFCE module installs/enables sddm instead of lightdm; one greeter across
  every session choice, no lightdm left anywhere.
- Interactive menus now default to 'none' for both DE and TWM (press Enter
  for no DE / no TWM). none+none is still refused by the existing guard.
- README and session.sh comments updated to match.

README.md

@@ -61,7 +61,7 @@ Each module pulls in `lib/` by relative path, so it works both ways.
 |---|---|---|
 | `gnome` | `gnome-desktop`, `gnome-desktop-environment`, `workstation-product-environment` | gdm |
 | `plasma` | `kde-desktop`, `kde-desktop-environment` | sddm |
-| `xfce` | `xfce-desktop`, `xfce-desktop-environment` | lightdm |
+| `xfce` | `xfce-desktop`, `xfce-desktop-environment` | sddm |
 
 ### Tiling window managers
 
@@ -78,7 +78,7 @@ compositors — do not expect the same config story from it.
 ## Display manager policy
 
 * A **DE module** leaves the DE's own greeter alone (GNOME→gdm, Plasma→sddm,
-  XFCE→lightdm). It never fights a desktop over its own login screen.
+  XFCE→sddm). It never fights a desktop over its own login screen.
 * A **TWM module** enforces **SDDM**, because it is the only greeter that lists
   Wayland compositors *and* X11 sessions in one menu.
 * Either way `graphical.target` becomes the default.