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/README.md

@@ -0,0 +1,22 @@
+# My Nomarchy machine
+
+1. `nixos-generate-config --show-hardware-config > hardware-configuration.nix`
+2. Edit `flake.nix` (Nomarchy repo URL, username), `system.nix` (hostname,
+   user), `home.nix` (your packages).
+3. `git init && git add -A` — flakes only see tracked files, including
+   `theme-state.json`.
+4. System: `sudo nixos-rebuild switch --flake .#default`
+5. Desktop: `nix run home-manager -- switch --flake .#me`
+   (afterwards just `home-manager switch --flake .#me` — it's installed)
+6. Clone/symlink this directory to `~/.nomarchy` (or export
+   `NOMARCHY_PATH`) so `nomarchy-theme-sync` knows where the state lives.
+
+Day-to-day:
+
+```sh
+nomarchy-theme-sync list             # 21 shipped presets
+nomarchy-theme-sync apply gruvbox    # writes state + home-manager switch
+nomarchy-theme-sync bg next          # cycle wallpapers (instant, no rebuild)
+```
+
+The system layer only needs `nixos-rebuild` when you change `system.nix`.