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/Pages/DiviUpdateAbility.php

@@ -41,7 +41,7 @@ final class DiviUpdateAbility extends DiviWriteAbility
     {
         return [
             'type' => 'object',
-            'properties' => array_merge(DiviWriter::inputProperties(), [
+            'properties' => array_merge(DiviSchema::write(), [
                 'changes' => [
                     'type' => 'array',
                     'minItems' => 1,