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/home.nix

@@ -0,0 +1,16 @@
+# Your user environment. The Nomarchy desktop (Hyprland, Waybar,
+# Ghostty, theming engine, Stylix) comes from homeModules.nomarchy;
+# tune it via the nomarchy.* options, add your own packages and
+# programs below.
+{ pkgs, ... }:
+
+{
+  # Examples:
+  # nomarchy.terminal = "kitty";        # swap the default terminal
+  # nomarchy.waybar.enable = false;     # bring your own bar
+  # nomarchy.stylix.enable = false;     # opt out of GTK/Qt theming
+
+  home.packages = with pkgs; [
+    # firefox
+  ];
+}