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>

README.md

@@ -39,7 +39,7 @@ Flat on purpose. Two module trees, one options file each, no hidden layers.
 ├── themes/                        # 24 presets: <slug>.json + optional <slug>/ assets
 │   ├── nord.json                  #   palette (required, works alone)
 │   └── nord/                      #   assets (optional, fixed filenames)
-│       ├── backgrounds/           #     wallpapers (auto-picked, SUPER+SHIFT+T cycles)
+│       ├── backgrounds/           #     wallpapers (pinned nomarchy-wallpapers input, not this repo)
 │       ├── btop.theme             #     hand-made config drop (else generated)
 │       └── waybar.css             #     whole-swap: replaces the generated bar style
 ├── modules/
@@ -434,7 +434,7 @@ per theme — a single place to look, unlike the old distro's split:
 
 | Asset | Mechanism |
 |---|---|
-| `backgrounds/` | wallpapers; empty `wallpaper` in the state means "first one"; `bg next` cycles |
+| `backgrounds/` | wallpapers; empty `wallpaper` in the state means "first one"; `bg next` cycles. Pinned in a separate `nomarchy-wallpapers` flake input, not this repo (ROADMAP § "Faster switches") — merged in at build time |
 | `btop.theme` | baked into the generation (generated from the palette when absent) |
 | `waybar.css` | **whole-swap**: replaces the generated bar style entirely (probed at eval time, self-contained) |
 | `waybar.jsonc` | whole-swap for the bar *layout* (must be plain JSON) |
@@ -443,7 +443,9 @@ per theme — a single place to look, unlike the old distro's split:
 Four themes ship a `waybar.css` identity (summer-day, summer-night,
 executive-slate, boreal). Custom user themes can live in
 `$NOMARCHY_PATH/themes/` (preset lookup) and `nomarchy.themesDir` (eval-time
-asset probe).
+asset probe) — including their own `backgrounds/`, which wins over the
+pinned `nomarchy-wallpapers` input (checked first); an explicit `wallpaper`
+path in `state.json` works too.
 
 ## 5. Day-to-day