feat: lit/tb-assign – conditions with dry run on live templates, reattach

- ThemeBuilderAccess::changeableTemplate(): rights (Q18 not_found), group gate,
  no TB draft, never the default template, current template_rev
- omitted exclude_from stays, [] clears (Q3); live dry run shows use_on and
  exclude_from before/after, overlaps and possible overlaps (B9)
- reattach only with published layouts (B10) and a live TB post; always
  disabled, unmarks template and layouts; failure restores everything

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Integrations/Divi/ThemeBuilder/ThemeBuilderAccess.php

@@ -154,6 +154,23 @@ class ThemeBuilderAccess
         return $forWrite ? ($this->writableGate() ?? $post) : $post;
     }
 
+    /**
+     * A template the Theme Builder tools may change (lit/tb-assign, lit/tb-activate): rights and group gate
+     * (template(), a template the user may not edit reads as missing – ruling Q18), no Theme Builder draft (E7),
+     * never the default template (E8), the current template_rev.
+     *
+     * @param array<string, mixed> $input template_id, template_rev
+     */
+    public function changeableTemplate(array $input): Template|WP_Error
+    {
+        $post = $this->template($input['template_id'] ?? null, true);
+        if ($post instanceof WP_Error) {
+            return $post;
+        }
+
+        return $this->checkedTemplate((int) $post->ID, $input);
+    }
+
     /**
      * Whether template $id may be named to this user (tb-list shows other IDs not at all, ruling Q8/B11).
      * $id must come from Divi's own template list (templateIds()/liveTemplates()/detachedIds()): this checks
@@ -244,4 +261,42 @@ class ThemeBuilderAccess
             'Im Theme Builder (Divi → Theme Builder) speichern oder die Änderungen verwerfen, dann erneut aufrufen.'
         );
     }
+
+    /**
+     * The draft, default and template_rev checks of changeableTemplate() for template $id (rights already checked).
+     *
+     * @param array<string, mixed> $input
+     */
+    private function checkedTemplate(int $id, array $input): Template|WP_Error
+    {
+        $refusal = $this->draftRefusal();
+        if ($refusal !== null) {
+            return $refusal;
+        }
+
+        $attached = in_array($id, $this->store->templateIds($this->store->liveId()), true);
+        $template = $this->store->template($id, $attached);
+        if ($template === null) {
+            return Errors::notFound(
+                sprintf('Theme-Builder-Vorlage %d gibt es nicht.', $id),
+                'Vorlagen mit lit/tb-list auflisten.'
+            );
+        }
+
+        if ($template->default) {
+            return Errors::unsupported(
+                'Die Standardvorlage ändert LIT MCP nicht; nichts wurde gespeichert.',
+                'Die Standardvorlage im Divi Theme Builder bearbeiten.'
+            );
+        }
+
+        if (! is_string($input['template_rev'] ?? null) || $input['template_rev'] !== $template->rev()) {
+            return Errors::revMismatch(
+                'Die Vorlage wurde seit dem Lesen geändert (template_rev passt nicht); nichts wurde gespeichert.',
+                'Mit lit/tb-list neu lesen und mit der neuen template_rev wiederholen.'
+            );
+        }
+
+        return $template;
+    }
 }