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/DynamicContentMapTest.php

@@ -61,12 +61,13 @@ final class DynamicContentMapTest extends TestCase
         self::assertSame('select', $map['post_date']['date_format']);
         self::assertSame('text', $map['post_excerpt']['words']);
 
-        $placeholders = new Placeholders(DiviFixtures::designSystem(), $map);
+        $placeholders = new Placeholders(DiviFixtures::designSystem());
+        $references = new VariableRef(DiviFixtures::designSystem(), $map);
         foreach (array_keys(Placeholders::POST) as $key) {
             $reference = $placeholders->resolve('{{post:' . $key . '}}');
             self::assertSame(
                 $reference,
-                $placeholders->references()->canonical($reference, ComposeFixture::sanitizer()),
+                $references->canonical($reference, ComposeFixture::sanitizer()),
                 $key
             );
         }