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>

themes/summer-day.json

@@ -0,0 +1,39 @@
+{
+  "version": 1,
+  "name": "Summer Day",
+  "slug": "summer-day",
+  "mode": "light",
+  "wallpaper": "",
+  "colors": {
+    "base": "#fdf6e3",
+    "mantle": "#d7d1c1",
+    "surface": "#5c6a72",
+    "overlay": "#829181",
+    "text": "#5c6a72",
+    "subtext": "#fdf6e3",
+    "muted": "#829181",
+    "accent": "#3a94c5",
+    "accentAlt": "#df69ba",
+    "good": "#8da101",
+    "warn": "#dfa000",
+    "bad": "#f85552"
+  },
+  "ansi": [
+    "#5c6a72",
+    "#f85552",
+    "#8da101",
+    "#dfa000",
+    "#3a94c5",
+    "#df69ba",
+    "#35a77c",
+    "#fdf6e3",
+    "#829181",
+    "#f85552",
+    "#8da101",
+    "#dfa000",
+    "#3a94c5",
+    "#df69ba",
+    "#35a77c",
+    "#fdf6e3"
+  ]
+}