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/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')));