gaming: add Steam/Sunshine stack + user-space toolkit

Host (hosts/common/features/gaming/):
- move steam.nix under gaming/, add gamemode + gamescope (capSysNice)
- sunshine.nix: Sunshine stream host for Moonlight/Nvidia Shield
  (openFirewall, capSysAdmin for Wayland/KMS capture, autoStart)

Home (home/common/programs/gaming.nix):
- reusable custom.gaming module: Lutris, Heroic, ProtonPlus,
  protontricks, MangoHud, vkbasalt, vulkan-tools
- NVIDIA-specific extras isolated behind custom.gaming.nvidia

Wire konishi: host imports ../gaming; home enables custom.gaming
with nvidia = true.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hosts/common/features/gaming/steam.nix

@@ -0,0 +1,10 @@
+_:
+
+{
+  programs.steam = {
+    enable = true;
+    remotePlay.openFirewall = true;
+  };
+
+  hardware.steam-hardware.enable = true;
+}