steamos: disable steam deck cmdlineConfig for non-steamdeck hosts

modules/desktop-jovian.nix

@@ -31,6 +31,15 @@
     user = username;
   };
 
+  # default off: jovian's SteamOS kernel cmdline (Valve's Deck-tuned amdgpu
+  # params: amd_iommu=off, amdgpu.sched_hw_submission=4, dcdebugmask=0x20000,
+  # lockup_timeout=5000,...,5000, ttm.pages_min=2097152, audit=0) is tuned for
+  # Van Gogh / Sephiroth APUs at 1280x800 and corrupts host memory pages on
+  # RDNA3 desktop GPUs (yarn: Navi 32 1440p, FH5 wine-log showed >1M
+  # illegal-instruction faults across .text after extended play).
+  # mkDefault so actual Deck hosts can opt back in (see hosts/patiodeck).
+  jovian.steamos.enableDefaultCmdlineConfig = lib.mkDefault false;
+
   # jovian overrides the display manager; sddm is required
   services.displayManager.sddm.wayland.enable = true;