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>

home/common/programs/gaming.nix

@@ -0,0 +1,72 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# User-space gaming toolkit. Vendor-agnostic on purpose so it can be reused on
+# AMD/Intel hosts — the only GPU-vendor-aware branch is `cfg.nvidia`, kept
+# deliberately small. The host still owns the privileged pieces (Steam FHS
+# wrapper, Sunshine, gamemode, gamescope caps) under
+# hosts/common/features/gaming.
+let
+  cfg = config.custom.gaming;
+in
+{
+  options.custom.gaming = {
+    enable = lib.mkEnableOption "user-space gaming tools (launchers, Proton mgmt, MangoHud)";
+
+    nvidia = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Enable NVIDIA-specific extras (currently just nvtop). Keep this the only
+        NVIDIA-aware branch in this file so the module stays reusable on other
+        GPUs. NVENC streaming needs no user config — it comes from the host
+        NVIDIA driver.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # NOTE: Steam itself is enabled at the host level (programs.steam) because it
+    # needs the FHS wrapper, udev rules and 32-bit graphics — home-manager can't
+    # provide those. See hosts/common/features/gaming/steam.nix.
+    home.packages =
+      with pkgs;
+      [
+        # launchers / stores
+        lutris
+        heroic # Epic / GOG / Amazon
+
+        # Proton / Wine compatibility tooling
+        protonplus # install & manage Proton-GE builds
+        protontricks
+        winetricks
+
+        # overlays, tuning & diagnostics
+        goverlay # GUI to configure MangoHud / vkBasalt
+        vkbasalt # Vulkan post-processing layer
+        vulkan-tools # vulkaninfo / vkcube for sanity checks
+      ]
+      ++ lib.optionals cfg.nvidia [
+        nvtopPackages.nvidia # GPU/VRAM/encoder monitor
+      ];
+
+    # In-game performance overlay (toggle with Shift+F12 by default).
+    programs.mangohud = {
+      enable = true;
+      settings = {
+        fps_limit = 0;
+        gpu_stats = true;
+        gpu_temp = true;
+        vram = true;
+        cpu_stats = true;
+        cpu_temp = true;
+        ram = true;
+        frametime = true;
+        frame_timing = true;
+      };
+    };
+  };
+}