feat: Nomarchy ground-up rewrite on NixOS 26.05

Full replacement of the previous iteration, rebuilt around three ideas:

- Pure evaluation: theme-state.json lives inside the flake and is read
  via the nomarchy.stateFile option — no --impure, ever.
- All-Home-Manager theming: `nomarchy-theme-sync apply` writes the JSON
  and runs `home-manager switch`; every theme change is one atomic,
  rollbackable generation. Wallpaper (swww) is the sole runtime piece.
- Flat, downstream-first layout: modules/{nixos,home} with one
  options.nix each, exported as nixosModules/homeModules + overlay +
  flake template; system (nixos-rebuild) and desktop (home-manager
  switch) rebuild paths are fully split.

Ships 21 themes imported from the previous iteration (palettes,
wallpapers, btop themes, six whole-swap Waybar identities), Stylix for
the GTK/Qt/cursor long tail, a live ISO target with offline theme
switching, and docs/TESTING.md with the QEMU verification workflow.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/downstream/system.nix

@@ -0,0 +1,25 @@
+# Your machine: bootloader, hostname, users, services.
+# The distro itself comes from nomarchy.nixosModules.nomarchy — override
+# any of its defaults here with plain NixOS options (they use mkDefault),
+# or via the nomarchy.system.* toggles.
+{ pkgs, ... }:
+
+{
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  networking.hostName = "my-nomarchy";
+  time.timeZone = "UTC";
+  i18n.defaultLocale = "en_US.UTF-8";
+
+  users.users.me = { # <- keep in sync with `username` in flake.nix
+    isNormalUser = true;
+    extraGroups = [ "wheel" "networkmanager" "video" "input" ];
+  };
+
+  # Examples:
+  # nomarchy.system.greeter.enable = false;   # bring your own login manager
+  # environment.systemPackages = [ pkgs.htop ];
+
+  system.stateVersion = "26.05";
+}