refactor: one VariableRef per composition, design lookups in DesignSystem (H3)

- DiviComposition::references() builds the one VariableRef with the entry's
  live map (S22/S31) and passes it to AttrWriter; Placeholders only needs the
  design system now, Placeholders::references() is gone (no reach-through).
- DesignSystem::colorIdFor()/variableFor() replace the dead colorId()/variable()
  and carry Placeholders' lookup incl. the S13a ambiguity check (byLabel moved,
  messages unchanged); isColorVariable() decides color vs content for {{var:}}.
- DesignSystem::colorReferenceIds() (VariableRef color names) and
  contentVariableIds() (DynamicContentRules design names) replace the two
  inline computations; same lists, same order.
Tests: fixtures/tests get the VariableRef from ComposeFixture::references() or
build it with the map; DesignSystemTest uses the new lookups (+2 tests).

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/DiviWriterTest.php

@@ -91,7 +91,7 @@ final class DiviWriterTest extends TestCase
         $writer = $this->writer(Mockery::mock(DiviSaver::class), FakeChangeLog::forbidden());
         $page = $writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)]);
 
-        $writer->composition($page)->placeholders();
+        $writer->composition($page)->references();
 
         self::assertSame([7], $this->registryCalls, 'the live dynamic content map of post 7 (ruling S22)');
     }