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

tests/Unit/Integrations/Divi/Pages/DiviChangeTest.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Pages;
+
+use LIT\MCP\Integrations\Divi\BlockTree\Block;
+use LIT\MCP\Integrations\Divi\BlockTree\NodeIndex;
+use LIT\MCP\Integrations\Divi\BlockTree\Parser;
+use LIT\MCP\Integrations\Divi\Pages\DiviChange;
+use PHPUnit\Framework\TestCase;
+
+final class DiviChangeTest extends TestCase
+{
+    public function testIdOfAnEntryAfterTheChange(): void
+    {
+        $index = NodeIndex::build((new Parser())->parse(
+            '<!-- wp:divi/section --><!-- wp:divi/row /--><!-- /wp:divi/section -->'
+        ));
+        $row = $index->find('s0.r0');
+        $change = (new DiviChange('x'))
+            ->changed($row, 'updated')
+            ->removed('s3.r1', Block::create('divi/row'))
+            ->changed(Block::create('divi/text'), 'inserted');
+
+        self::assertSame(
+            ['s0.r0', 's3.r1', null],
+            array_map(static fn (array $entry): ?string => DiviChange::idOf($entry, $index), $change->entries())
+        );
+    }
+}