fix: drop mkDefault on three more list options it was silently dropping

Same footgun as the home.packages fix, found by sweeping the tree for
`mkDefault` on list options that other modules contribute to at normal
priority (so filterOverrides discards the mkDefault def entirely):

  * themes/engine/sddm.nix — environment.systemPackages = mkDefault
    [ nomarchy-sddm-theme ] was dropped, so the SDDM greeter theme package
    was never installed (unthemed login).
  * themes/engine/plymouth.nix — boot.kernelParams = mkDefault [ quiet
    splash loglevel=3 … ] was dropped, so boots weren't quiet/clean.
  * features/desktop/waybar/default.nix — home.packages = mkDefault
    [ font-awesome ] was dropped, so waybar's icon font was missing.

Verified via eval: nomarchy-sddm-theme now in systemPackages, "quiet" in
kernelParams, font-awesome in home.packages. Left the genuinely-safe
single-definition mkDefaults alone (plymouth.themePackages,
resolved.fallbackDns, hyprsunset.extraArgs) and the hybridGPU videoDrivers
mkDefault (it outranks nixpkgs' mkOptionDefault on real hardware).

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

features/desktop/waybar/default.nix

@@ -47,7 +47,10 @@ in
     style = lib.mkDefault (builtins.readFile styleFile);
   };
 
-  home.packages = lib.mkDefault (with pkgs; [
+  # Not mkDefault: home.packages is a list other modules set at normal
+  # priority, so a mkDefault def is dropped — leaving font-awesome (waybar's
+  # icon font) uninstalled. Merge it in.
+  home.packages = with pkgs; [
     font-awesome
-  ]);
+  ];
 }

themes/engine/plymouth.nix

@@ -70,5 +70,8 @@ in
     theme = lib.mkDefault "nomarchy";
   };
 
-  boot.kernelParams = lib.mkDefault [ "quiet" "splash" "loglevel=3" "rd.systemd.show_status=false" "rd.udev.log_level=3" "udev.log_priority=3" "boot.shell_on_fail" ];
+  # Not mkDefault: kernelParams is a list nixpkgs and other modules add to at
+  # normal priority, so a mkDefault def is dropped — losing the quiet/splash
+  # boot. These params merge with (don't replace) the rest.
+  boot.kernelParams = [ "quiet" "splash" "loglevel=3" "rd.systemd.show_status=false" "rd.udev.log_level=3" "udev.log_priority=3" "boot.shell_on_fail" ];
 }

themes/engine/sddm.nix

@@ -36,7 +36,10 @@ in
     user = lib.mkDefault "nomarchy";
   };
 
-  environment.systemPackages = lib.mkDefault [ nomarchy-sddm-theme ];
+  # Not mkDefault: systemPackages is a list every module contributes to at
+  # normal priority, so a mkDefault def is dropped entirely — leaving the
+  # SDDM theme package uninstalled and the greeter unthemed.
+  environment.systemPackages = [ nomarchy-sddm-theme ];
 
   # Enable Hyprland system-level dependencies
   programs.hyprland = {