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/DiviCompositionTest.php

@@ -71,6 +71,7 @@ final class DiviCompositionTest extends TestCase
         self::assertSame([7], $this->registryCalls, 'the registry is read once per post');
         self::assertSame($composition->attrWriter(), $composition->attrWriter());
         self::assertSame($composition->placeholders(), $composition->placeholders());
+        self::assertSame($composition->references(), $composition->references());
     }
 
     public function testWithoutAPostTheDefaultMapApplies(): void
@@ -97,6 +98,8 @@ final class DiviCompositionTest extends TestCase
         self::assertNotSame($base, $seven);
         self::assertNotSame($seven->placeholders(), $eight->placeholders());
         self::assertNotSame($base->placeholders(), $seven->placeholders());
+        self::assertNotSame($seven->references(), $eight->references());
+        self::assertNotSame($base->references(), $seven->references());
         self::assertSame([7, 8], $this->registryCalls);
     }