feat(apps): starter workstation app suite + steam/libvirt/obs services

The "default application suite" roadmap item, but deliberately NOT a
nomarchy.apps.* option surface: a bare package install has no config to
put behind a toggle, so a thin `mkIf x { systemPackages = [p]; }` only
reinvents what a package list already gives. Instead the suite is a
curated home.packages list in templates/downstream/home.nix (extending
the existing `# firefox` convention) — active staples (libreoffice,
vscode, gimp, inkscape, mpv, amberol) plus commented suggestions. The
opt-out is deleting a line; it also keeps the apps out of the live ISO
automatically (the ISO never scaffolds from the template).

The members that DO need system config become nomarchy.services.* opt-in
toggles instead (config behind the switch, like the existing services):
  - steam   → programs.steam (32-bit stack, controller udev, RP ports)
  - libvirt → libvirtd + virt-manager, login user added to libvirtd group
  - obs     → obs-studio + a v4l2loopback virtual camera (exclusive_caps=1)
              selectable as a webcam in Zoom/Teams

README option table, ROADMAP (services item now "Eight shipped"; suite
item marked done with the no-surface rationale) and the downstream
system.nix examples updated to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/services.nix

@@ -29,6 +29,22 @@ in
     pika.enable = lib.mkEnableOption ''
       Pika Backup, a friendly GUI for scheduled Borg backups (the app;
       configure repositories and schedules inside it)'';
+
+    steam.enable = lib.mkEnableOption ''
+      Steam (Valve's game store/launcher) via programs.steam — which wires up
+      the 32-bit graphics libraries, controller udev rules and Remote-Play
+      firewall ports a bare package can't. Unfree'';
+
+    libvirt.enable = lib.mkEnableOption ''
+      libvirt/KVM virtualisation with the virt-manager GUI — runs the
+      libvirtd daemon and adds the login user to the `libvirtd` group, so VMs
+      can be managed without root'';
+
+    obs.enable = lib.mkEnableOption ''
+      OBS Studio for screen recording and streaming, plus a v4l2loopback
+      virtual camera — so an OBS scene can be selected as a webcam in Zoom,
+      Teams or a browser (screen capture itself already works over the
+      desktop's PipeWire portal)'';
   };
 
   config = lib.mkMerge [
@@ -80,5 +96,31 @@ in
     (lib.mkIf cfg.pika.enable {
       environment.systemPackages = [ pkgs.pika-backup ];
     })
+
+    (lib.mkIf cfg.steam.enable {
+      # The Steam package alone misses the system bits a game needs: the
+      # 32-bit graphics stack, controller udev rules, Remote-Play ports.
+      programs.steam.enable = true;
+    })
+
+    (lib.mkIf cfg.libvirt.enable {
+      virtualisation.libvirtd.enable = true;
+      programs.virt-manager.enable = true;
+      # Group membership lets the user reach the system libvirtd socket
+      # (manage VMs) without sudo. Merges with the user's other extraGroups.
+      users.users.${args.username}.extraGroups = [ "libvirtd" ];
+    })
+
+    (lib.mkIf cfg.obs.enable {
+      environment.systemPackages = [ pkgs.obs-studio ];
+      # Virtual camera: OBS streams a scene into a v4l2 loopback device that
+      # Zoom/Teams/browsers then pick up as a webcam. exclusive_caps=1 is the
+      # bit that makes those apps actually list the device.
+      boot.extraModulePackages = [ config.boot.kernelPackages.v4l2loopback ];
+      boot.kernelModules = [ "v4l2loopback" ];
+      boot.extraModprobeConfig = ''
+        options v4l2loopback devices=1 video_nr=10 card_label="OBS Virtual Camera" exclusive_caps=1
+      '';
+    })
   ];
 }