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.lock

@@ -212,6 +212,23 @@
         "type": "github"
       }
     },
+    "nomarchy-wallpapers": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1784302860,
+        "narHash": "sha256-nQ+eLEggLxDcbKks3Ui6rbJ4P24H2JXenLLg9si007M=",
+        "ref": "main",
+        "rev": "803b48ddf35aeff08999c8c7f563457181105cf5",
+        "revCount": 2,
+        "type": "git",
+        "url": "https://git.bemagri.xyz/bernardo/Nomarchy-Wallpapers.git"
+      },
+      "original": {
+        "ref": "main",
+        "type": "git",
+        "url": "https://git.bemagri.xyz/bernardo/Nomarchy-Wallpapers.git"
+      }
+    },
     "nur": {
       "inputs": {
         "flake-parts": [
@@ -243,6 +260,7 @@
         "nixos-hardware": "nixos-hardware",
         "nixpkgs": "nixpkgs",
         "nixpkgs-unstable": "nixpkgs-unstable",
+        "nomarchy-wallpapers": "nomarchy-wallpapers",
         "stylix": "stylix"
       }
     },