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

src/Integrations/Divi/Compose/DynamicContentRules.php

@@ -116,13 +116,7 @@ final class DynamicContentRules
             ));
         }
 
-        $design = array_values(DesignSystem::FONT_VARIABLES);
-        foreach ($this->design->variables() as $variable) {
-            if ($variable['type'] !== 'colors') {
-                $design[] = $variable['id'];
-            }
-        }
-        if (in_array($name, $design, true)) {
+        if (in_array($name, $this->design->contentVariableIds(), true)) {
             return self::DESIGN_SETTINGS;
         }