feat: implement hybrid declarative state with automatic Nix sync

themes/engine/scripts/nomarchy-theme-bg-next

@@ -45,8 +45,7 @@ done
 NEXT_INDEX=$(((INDEX + 1) % TOTAL))
 NEW_BG="$BG_DIR/${BACKGROUNDS[$NEXT_INDEX]}"
 
-TMP_JSON=$(mktemp)
-jq --arg wp "$NEW_BG" '.wallpaper = $wp' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
+nomarchy-state-write wallpaper "$NEW_BG"
 
 # Instant feedback via swww
 if pgrep -x swww-daemon >/dev/null; then