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>

tests/Unit/Integrations/Divi/DiviIntegrationTest.php

@@ -78,7 +78,7 @@ final class DiviIntegrationTest extends TestCase
         self::assertSame('Divi Theme Builder', $group->label());
         self::assertFalse($group->isAvailable(), 'no Divi classes in unit tests');
         self::assertSame(
-            ['lit/tb-list', 'lit/tb-create-template'],
+            ['lit/tb-list', 'lit/tb-create-template', 'lit/tb-assign'],
             array_map(static fn (Ability $ability): string => $ability->name(), $group->abilities())
         );
         foreach ($group->abilities() as $ability) {