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/BlockTree/NodeIndex.php

@@ -36,7 +36,7 @@ final class NodeIndex
     {
         $root = $document;
         foreach ($document->children() as $block) {
-            if ($block->name() === 'divi/placeholder') {
+            if ($block->isPlaceholder()) {
                 $root = $block;
                 break;
             }
@@ -92,6 +92,26 @@ final class NodeIndex
         return $parent instanceof Block ? $this->idOf($parent) : null;
     }
 
+    /** The parent node of $block; null at the top level (the root or a placeholder holds it). */
+    public function parentBlock(Block $block): ?Block
+    {
+        $parent = $this->parentOf($block);
+
+        return $parent instanceof Block && ! $parent->isPlaceholder() ? $parent : null;
+    }
+
+    /** Admin label of the section around $block (or $block itself), for automatic labels ("" when none). */
+    public function sectionLabel(?Block $block): string
+    {
+        for ($node = $block; $node !== null; $node = $this->parentBlock($node)) {
+            if ($node->name() === 'divi/section') {
+                return NodeSummary::label($node);
+            }
+        }
+
+        return '';
+    }
+
     /** @return list<array{id: string, block: Block, depth: int}> In document order. */
     public function entries(): array
     {
@@ -102,7 +122,7 @@ final class NodeIndex
     private function walk(Container $container, string $prefix, int $depth, array &$counters): void
     {
         foreach ($container->children() as $block) {
-            if ($prefix === '' && $block->name() === 'divi/placeholder') {
+            if ($prefix === '' && $block->isPlaceholder()) {
                 $this->walk($block, '', $depth, $counters);
                 continue;
             }