Add gaming.nix for ringtail: gamescope + consolidate Steam config

Move Steam config from configuration.nix to a dedicated gaming.nix module
and add gamescope for fullscreen/resolution management with Proton games.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nixos/ringtail/configuration.nix

@@ -113,12 +113,6 @@ in
     polkitPolicyOwners = [ "eblume" ];
   };
 
-  # Steam
-  programs.steam = {
-    enable = true;
-    dedicatedServer.openFirewall = true;
-  };
-
   # K3s single-node cluster
   services.k3s = {
     enable = true;

nixos/ringtail/flake.nix

@@ -28,6 +28,7 @@
         ./disk-config.nix
         ./hardware-configuration.nix
         ./configuration.nix
+        ./gaming.nix
         # Pin versioned services to nixpkgs-services instead of the rolling nixpkgs.
         # This prevents `nix flake update nixpkgs` from silently upgrading them.
         # Bump nixpkgs-services explicitly during service reviews.

nixos/ringtail/gaming.nix

@@ -0,0 +1,16 @@
+{ pkgs, ... }:
+
+{
+  # Steam
+  programs.steam = {
+    enable = true;
+    dedicatedServer.openFirewall = true;
+  };
+
+  # Gamescope — micro-compositor for game fullscreen/resolution management.
+  # Use as Steam launch option: gamescope -W 2560 -H 1440 -f -- %command%
+  programs.gamescope = {
+    enable = true;
+    capSysNice = true; # Allow gamescope to set realtime scheduling
+  };
+}