fix: tb-assign reattaches only templates no Theme Builder post lists (audit L4)

"Detached" now means for lit/tb-assign what lit/tb-list shows as detached:
DiviThemeBuilder::isListedAnywhere() looks for any _et_template row (live,
draft or Theme Builder library post). A template another Theme Builder post
lists is refused with or without reattach; nothing is written.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/ThemeBuilder/TbAssignAbility.php

@@ -23,7 +23,8 @@ defined('ABSPATH') || exit;
  * (ruling Q3/B3). A live template changes the website at once, so it is a dry run first and needs confirm: true
  * (ruling Q9/B22), listing both lists before/after and overlaps (identical and possible, B9) with other live
  * templates for the resulting lists. reattach: true puts a template a stale Theme Builder save removed back on the
- * live list (only on the user's request, B19) – always disabled, Divi's "unused" marks removed from it and its
+ * live list (only on the user's request, B19; only one no Theme Builder post lists – tb-list's detached, audit L4)
+ * – always disabled, Divi's "unused" marks removed from it and its
  * layouts, which must still be published (B10). Templates have no revisions: the previous conditions are in the
  * answer and the change log; a failed write restores them (and undoes the reattach).
  */
@@ -144,13 +145,30 @@ final class TbAssignAbility implements Ability
         return $this->write($template, $useOn['ids'], $excludeFrom['ids'], $reattach);
     }
 
-    /** A detached template needs reattach: true, a live Theme Builder post and published layouts (B10). */
+    /**
+     * A detached template needs reattach: true, a live Theme Builder post and published layouts (B10). Detached
+     * means what tb-list shows as detached: no Theme Builder post lists it – a template of a Theme Builder library
+     * item or draft is none (audit L4).
+     */
     private function reattachRefusal(Template $template, bool $reattach): ?WP_Error
     {
         if ($template->attached) {
             return null;
         }
 
+        if ($this->access->store()->isListedAnywhere($template->id)) {
+            return Errors::invalidInput(
+                sprintf(
+                    'Vorlage %d ist keine entfernte Vorlage (siehe detached in lit/tb-list); nichts wurde gespeichert.',
+                    $template->id
+                ),
+                [
+                    'template_id' => 'nur Vorlagen der Website (templates) oder entfernte Vorlagen (detached) aus '
+                        . 'lit/tb-list',
+                ]
+            );
+        }
+
         if (! $reattach) {
             $marked = $template->markedUnused === null
                 ? ''