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/BlockTree/BlockTest.php

@@ -38,6 +38,13 @@ final class BlockTest extends TestCase
         self::assertSame('divi/button', Block::create('divi/button')->name());
     }
 
+    public function testOnlyThePlaceholderIsAPlaceholder(): void
+    {
+        self::assertTrue(Block::create('divi/placeholder')->isPlaceholder());
+        self::assertFalse(Block::create('divi/section')->isPlaceholder());
+        self::assertFalse(Block::create('core/placeholder')->isPlaceholder());
+    }
+
     public function testSetAttrsRefusesOnAnOpaqueGlobalModule(): void
     {
         $document = (new Parser())->parse('<!-- wp:divi/section {"globalModule":"x"} --><!-- /wp:divi/section -->');

tests/Unit/Integrations/Divi/BlockTree/NodeIndexTest.php

@@ -45,6 +45,20 @@ final class NodeIndexTest extends TestCase
         self::assertSame(0, preg_match(NodeIndex::ID_PATTERN, "s1\n"));
     }
 
+    public function testParentBlockSkipsThePlaceholderAndSectionLabelWalksUp(): void
+    {
+        $index = NodeIndex::build((new Parser())->parse($this->fixture('synthetic-basic.html')));
+        $button = $index->find('s0.r0.c1.m2');
+
+        self::assertTrue($index->root() instanceof \LIT\MCP\Integrations\Divi\BlockTree\Block
+            && $index->root()->isPlaceholder());
+        self::assertSame($index->find('s0.r0.c1'), $index->parentBlock($button));
+        self::assertNull($index->parentBlock($index->find('s0')), 'the placeholder is no parent node');
+        self::assertSame('Hero', $index->sectionLabel($button));
+        self::assertSame('Hero', $index->sectionLabel($index->find('s0')));
+        self::assertSame('', $index->sectionLabel(null));
+    }
+
     public function testDocumentWithoutPlaceholderIsItsOwnRoot(): void
     {
         $document = (new Parser())->parse('<!-- wp:divi/section /-->');

tests/Unit/Integrations/Divi/BlockTree/OutlineTest.php

@@ -82,6 +82,22 @@ final class OutlineTest extends TestCase
         self::assertTrue($outline->wasTruncated());
     }
 
+    public function testSummaryOfThePageOrOfSections(): void
+    {
+        $index = NodeIndex::build((new Parser())->parse($this->fixture('synthetic-basic.html')));
+        $outline = new Outline();
+
+        self::assertSame(
+            ['text' => $outline->text($outline->rows($index)), 'truncated' => false],
+            $outline->summary($index)
+        );
+        self::assertSame(
+            $outline->text($outline->rows($index, 's0')) . "\n" . $outline->text($outline->rows($index, 's1')),
+            $outline->summary($index, ['s0', 's1'])['text']
+        );
+        self::assertSame(['text' => null, 'truncated' => false], $outline->summary($index, []));
+    }
+
     public function testReadOnlyNodesAreMarked(): void
     {
         $index = NodeIndex::build((new Parser())->parse($this->fixture('synthetic-mixed.html')));