feat(themes): wallpapers split — 94 MB of runtime-only images leave the flake source

The split line is consumption time: palette JSONs, previews and
per-theme overrides are read at Nix eval and stay in-repo (~3 MB);
wallpapers are runtime-only (swww via nomarchy-state-sync, never read
at eval) and move to the pinned nomarchy-wallpapers non-flake input
(git.bemagri.xyz/bernardo/Nomarchy-Wallpapers @ 803b48d, all 24 slugs).

New overlay drv nomarchy-default-themes merges the two: repo themes
copied, <slug>/backgrounds symlinked from the input, build FAILS if any
theme JSON lacks non-empty backgrounds — the drift guard for the
two-repo dance. nomarchy-state-sync's wrapper now points straight at
the merged store path (no in-package copy): the package drops from
~94 MB to 32.6 KiB and the wallpapers source appears exactly once in
any system closure, ISO included — offline promise intact. Downstream
custom wallpapers still win ($NOMARCHY_PATH/themes is checked first),
and an explicit state.json wallpaper path still works.

checks re-pointed at the merged tree (theme-wholeswap,
state-sync-validate, auto-theme); theme-contrast and airplane stay on
./themes (JSON/text only). import-palettes.py + README updated;
ROADMAP § Faster switches marked shipped; LATER now carries only the
pre-built-variants follow-on. Also files #151 (v1 launch plan, [human])
with measured history numbers: the 107 MiB pack is 94 MB wallpaper
blobs; all 848 text commits pack to ~15-20 MiB — recommendation
recorded to filter-repo at the GitHub move, not fresh-start.

Verification: V2 — nix flake check --no-build green; theme-wholeswap,
state-sync-validate, auto-theme VM check, downstream-template-home all
green; wallpapers store path exactly once in the state-sync closure;
`list` works from a scratch state dir. V3 queued: live wallpaper render
+ bg-next + custom-override precedence after next pull (HARDWARE-QUEUE).
Lock gained exactly the one input node. Implementation by a Sonnet
subagent; design and review on Fable.

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

flake.nix

@@ -30,9 +30,20 @@
       url = "github:NixOS/nixos-hardware/master";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    # Wallpapers only — split out of this repo (ROADMAP § "Faster
+    # switches") so a `state.json` write stops re-copying ~94 MB of
+    # backgrounds/ before Nix can evaluate a 1 KB change. Runtime-only:
+    # never read at eval, only by nomarchy-state-sync's Python (swww
+    # path) via NOMARCHY_DEFAULT_THEMES. `flake = false` — it's just a
+    # content tree (`<slug>/backgrounds/*`), not a flake.
+    nomarchy-wallpapers = {
+      url = "git+https://git.bemagri.xyz/bernardo/Nomarchy-Wallpapers.git?ref=main";
+      flake = false;
+    };
   };
 
-  outputs = { self, nixpkgs, nixpkgs-unstable, home-manager, stylix, nixos-hardware, ... }:
+  outputs = { self, nixpkgs, nixpkgs-unstable, home-manager, stylix, nixos-hardware, nomarchy-wallpapers, ... }:
     let
       system = "x86_64-linux";
       username = "nomarchy"; # reference host only; downstream picks its own
@@ -95,10 +106,43 @@
           inherit (prev.stdenv.hostPlatform) system;
           config.allowUnfree = true;
         };
+        # The default themes root: this repo's ./themes (JSONs, previews,
+        # btop/waybar/rofi overrides — ~3 MB post wallpaper-split) merged
+        # with backgrounds/ symlinked in from the pinned nomarchy-wallpapers
+        # input. One tree so nomarchy-state-sync (baked-in fallback) and
+        # the theme-wholeswap check both see slug/backgrounds/ in the same
+        # place a downstream machine would via NOMARCHY_DEFAULT_THEMES.
+        nomarchy-default-themes =
+          let
+            wallpaperSlugs = builtins.attrNames
+              (nixpkgs.lib.filterAttrs (n: t: t == "directory")
+                (builtins.readDir nomarchy-wallpapers));
+            linkBackgrounds = slug: ''
+              if [ -d ${nomarchy-wallpapers}/${slug}/backgrounds ]; then
+                mkdir -p $out/${slug}
+                ln -s ${nomarchy-wallpapers}/${slug}/backgrounds $out/${slug}/backgrounds
+              fi
+            '';
+          in
+          final.runCommand "nomarchy-default-themes" { } ''
+            cp -r ${./themes} $out
+            chmod -R u+w $out
+
+            ${nixpkgs.lib.concatMapStringsSep "\n" linkBackgrounds wallpaperSlugs}
+
+            for j in $out/*.json; do
+              slug=$(basename "$j" .json)
+              if [ ! -d "$out/$slug/backgrounds" ] || [ -z "$(ls -A "$out/$slug/backgrounds" 2>/dev/null)" ]; then
+                echo "nomarchy-default-themes: $slug has no non-empty backgrounds/ (drift vs nomarchy-wallpapers)" >&2
+                exit 1
+              fi
+            done
+          '';
         nomarchy-state-sync = final.callPackage ./pkgs/nomarchy-state-sync {
           # Presets baked into the package, so `nomarchy-state-sync list`
-          # works on machines that don't check out this repo.
-          themesDir = ./themes;
+          # works on machines that don't check out this repo. Already a
+          # store artifact (nomarchy-default-themes) — no copy needed.
+          themesDir = final.nomarchy-default-themes;
         };
         nomarchy-doctor = final.callPackage ./pkgs/nomarchy-doctor { };
         nomarchy-battery-notify = final.callPackage ./pkgs/nomarchy-battery-notify { };
@@ -336,11 +380,13 @@
           # Whole-swap waybar.css is read raw (no palette prepend) — every
           # @color it references must be @define-color'd in the same file
           # (neon-glass shipped zero defines; BACKLOG #62). Also gate
-          # preview.png + non-empty backgrounds/ per preset.
+          # preview.png + non-empty backgrounds/ per preset — run against
+          # the merged tree (pkgs.nomarchy-default-themes) so this doubles
+          # as the drift guard between this repo and nomarchy-wallpapers.
           theme-wholeswap = pkgs.runCommand "nomarchy-theme-wholeswap"
             { nativeBuildInputs = [ pkgs.python3 ]; }
             ''
-              python3 ${./tools/check-theme-wholeswap.py} ${./themes}
+              python3 ${./tools/check-theme-wholeswap.py} ${pkgs.nomarchy-default-themes}
               touch $out
             '';
 
@@ -389,7 +435,7 @@
             ''
               set -eu
               tool=${./pkgs/nomarchy-state-sync/nomarchy-state-sync.py}
-              export NOMARCHY_DEFAULT_THEMES=${./themes}
+              export NOMARCHY_DEFAULT_THEMES=${pkgs.nomarchy-default-themes}
 
               mkdir good && cp ${./templates/downstream/state.json} good/state.json
               chmod +w good/state.json
@@ -2180,7 +2226,7 @@
               )
 
               env = ("NOMARCHY_PATH=/root/.nomarchy "
-                     "NOMARCHY_DEFAULT_THEMES=${./themes} "
+                     "NOMARCHY_DEFAULT_THEMES=${pkgs.nomarchy-default-themes} "
                      "NOMARCHY_REBUILD=/root/rebuild ")
 
               # Pair + schedule (setup writes only, no rebuild).