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>

modules/nixos/options.nix

@@ -0,0 +1,14 @@
+# System-level `nomarchy.system.*` options.
+#
+# Deliberately small: only things a downstream user plausibly disagrees
+# with get a toggle. Everything else in the system module is set with
+# lib.mkDefault, so plain NixOS options override it natively.
+{ lib, ... }:
+
+{
+  options.nomarchy.system = {
+    greeter.enable = lib.mkEnableOption "the greetd/tuigreet login screen" // { default = true; };
+    audio.enable = lib.mkEnableOption "the Pipewire audio stack" // { default = true; };
+    bluetooth.enable = lib.mkEnableOption "Bluetooth support with blueman" // { default = true; };
+  };
+}