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/Design/DiviDesignSystemAbilityTest.php

@@ -7,6 +7,7 @@ namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
 use Brain\Monkey\Functions;
 use LIT\MCP\Integrations\Divi\Design\DesignSystemReader;
 use LIT\MCP\Integrations\Divi\Design\DiviDesignSystemAbility;
+use LIT\MCP\Tests\Support\DiviFixtures;
 use LIT\MCP\Tests\Unit\TestCase;
 use Mockery;
 
@@ -15,7 +16,7 @@ final class DiviDesignSystemAbilityTest extends TestCase
     public function testReturnsDesignSystemWithPlaceholders(): void
     {
         $reader = Mockery::mock(DesignSystemReader::class);
-        $reader->allows('read')->andReturn(DesignSystemTest::sample());
+        $reader->allows('read')->andReturn(DiviFixtures::designSystem());
         Functions\when('current_user_can')->justReturn(true);
         $ability = new DiviDesignSystemAbility($reader);