refactor: small Divi tree helpers out of DiviWriter (M3)

Block::isPlaceholder replaces the divi/placeholder literals (ledger T4);
NodeIndex::parentBlock/sectionLabel replace the two parentBlock copies and
DiviWriter::sectionLabel; DiviChange::idOf and Outline::summary replace the
duplicated id resolution and outline text; DiviSchema holds the shared
input properties. Behaviour unchanged.

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

src/Integrations/Divi/Compose/TreeEditor.php

@@ -65,7 +65,7 @@ final class TreeEditor
 
         return [
             'container' => $container,
-            'parent' => $container instanceof Block && $container->name() !== 'divi/placeholder' ? $container : null,
+            'parent' => $container instanceof Block && ! $container->isPlaceholder() ? $container : null,
             'index' => $key === 'before' ? $sibling : $sibling + 1,
         ];
     }
@@ -96,7 +96,7 @@ final class TreeEditor
     /** @param array{container: Container, parent: ?Block, index: int} $position */
     public function move(NodeIndex $index, Block $node, array $position): ?string
     {
-        for ($ancestor = $position['parent']; $ancestor !== null; $ancestor = $this->parentBlock($index, $ancestor)) {
+        for ($ancestor = $position['parent']; $ancestor !== null; $ancestor = $index->parentBlock($ancestor)) {
             if ($ancestor === $node) {
                 return 'ein Knoten kann nicht in sich selbst verschoben werden';
             }
@@ -139,13 +139,6 @@ final class TreeEditor
         return $copy;
     }
 
-    private function parentBlock(NodeIndex $index, Block $block): ?Block
-    {
-        $parent = $index->parentOf($block);
-
-        return $parent instanceof Block && $parent->name() !== 'divi/placeholder' ? $parent : null;
-    }
-
     /**
      * Ruling S9: inserting at the document root of an empty page (no divi/placeholder yet)
      * first wraps it in one, the same way DiviContentComposer (Task 9) writes new content –