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/Outline.php

@@ -32,6 +32,29 @@ final class Outline
         return $this->truncated;
     }
 
+    /**
+     * The outline text of a write result: the whole page ($sectionIds null) or the given
+     * sections one after another (none: text null), and whether any breadth cap fired (S18).
+     *
+     * @param list<string>|null $sectionIds
+     * @return array{text: ?string, truncated: bool}
+     */
+    public function summary(NodeIndex $index, ?array $sectionIds = null): array
+    {
+        if ($sectionIds === []) {
+            return ['text' => null, 'truncated' => false];
+        }
+
+        $texts = [];
+        $truncated = false;
+        foreach ($sectionIds ?? [null] as $sectionId) {
+            $texts[] = $this->text($this->rows($index, $sectionId));
+            $truncated = $truncated || $this->wasTruncated();
+        }
+
+        return ['text' => implode("\n", $texts), 'truncated' => $truncated];
+    }
+
     /**
      * @return list<array{id: string, type: string, label: string, excerpt: string, info: string,
      *     presets: list<string>, children: int, editable: bool, depth: int, truncated: bool}>