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/Block.php

@@ -121,6 +121,12 @@ final class Block extends Container
         return str_starts_with($this->name, 'divi/');
     }
 
+    /** The divi/placeholder wrapper the builder puts around a page's sections (transparent for IDs). */
+    public function isPlaceholder(): bool
+    {
+        return $this->name === 'divi/placeholder';
+    }
+
     /** A global module reference or global layout (edited only in Etappe 3). */
     public function isGlobal(): bool
     {