test: shared Divi fixtures in tests/Support (T1)

- DiviFixtures: markup(), givenPost(), schema(), designSystem(), catalog(),
  dynamicContentRegistry(), maliciousRefs() - moved verbatim out of
  DiviPageLoaderTest, DesignSystemTest, ModuleCatalogTest, DynamicContentMapTest
  and VariableRefTest (its malicious() provider now returns maliciousRefs()).
- ComposeFixture moves to tests/Support.
- DiviWriterFixture: modules(), designReader(), composition(), a recording saver(),
  log() and writer(&$saved, &$failures, unfiltered, loader) for the five tests that
  wired the Divi write stack by hand.
No test cases or assertions changed (1397 / 5834).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/Divi/Compose/TreeEditorTest.php

@@ -10,6 +10,7 @@ use LIT\MCP\Integrations\Divi\BlockTree\NodeIndex;
 use LIT\MCP\Integrations\Divi\BlockTree\Parser;
 use LIT\MCP\Integrations\Divi\BlockTree\Serializer;
 use LIT\MCP\Integrations\Divi\Compose\TreeEditor;
+use LIT\MCP\Tests\Support\ComposeFixture;
 use PHPUnit\Framework\TestCase;
 
 final class TreeEditorTest extends TestCase