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>

tools/import-palettes.py

@@ -13,7 +13,12 @@ Old format (per theme directory):
 
 Output, per theme:
     <slug>.json            the palette (Nomarchy theme schema)
-    <slug>/backgrounds/    wallpapers, copied verbatim
+    <slug>/backgrounds/    wallpapers, copied verbatim — NOTE: this repo's
+                           own themes/ no longer carries backgrounds/ (moved
+                           to the Nomarchy-Wallpapers repo, ROADMAP § "Faster
+                           switches"); if <output-themes-dir> is this repo's
+                           themes/, move the copied backgrounds/ over there
+                           instead of committing them here
     <slug>/btop.theme      copied from apps/ when present
 
 Hierarchy vs ANSI (#70):
@@ -166,12 +171,16 @@ def main() -> None:
         slug = theme["slug"]
         (out / f"{slug}.json").write_text(json.dumps(theme, indent=2) + "\n")
 
-        # Assets: wallpapers and hand-made per-app theme files.
+        # Assets: wallpapers and hand-made per-app theme files. Wallpapers
+        # land in <out>/<slug>/backgrounds/ here for convenience, but if
+        # <out> is this repo's themes/, move them to the Nomarchy-Wallpapers
+        # repo instead of committing them (ROADMAP § "Faster switches") —
+        # this repo's themes/*/backgrounds/ was removed on purpose.
         extras = []
         if (theme_dir / "backgrounds").is_dir():
             shutil.copytree(theme_dir / "backgrounds", out / slug / "backgrounds",
                             dirs_exist_ok=True)
-            extras.append(f"{len(list((theme_dir / 'backgrounds').iterdir()))} wallpapers")
+            extras.append(f"{len(list((theme_dir / 'backgrounds').iterdir()))} wallpapers — move to Nomarchy-Wallpapers")
         if (theme_dir / "apps" / "btop.theme").is_file():
             (out / slug).mkdir(parents=True, exist_ok=True)
             shutil.copyfile(theme_dir / "apps" / "btop.theme", out / slug / "btop.theme")