refactor: split VariableRef into ControlChars and DynamicContentRules (H2)

- ControlChars::strip()/stripDeep() (was VariableRef::withoutControls/Deep) with
  the S32/S32a/S35 docblock; callers AttrWriter, ShortformWriter, TextReplacer and
  the setting cleaner use it.
- DynamicContentRules (design system + dynamic content map): fieldsFor() (was
  contentFields: meta prefix -> map -> loop_ -> design names -> unknown),
  cleanSetting() (was setting(): raw strip -> raw marker -> field type ->
  final-string check -> meta key check, same order), one private method per
  field family (textSetting, selectPostSetting, scalarSetting), isMetaKey(),
  checkMap(), and the constants DEFAULT_DYNAMIC_CONTENT, META_SETTINGS,
  DESIGN_SETTINGS, META_KEY.
- VariableRef keeps the syntax: const END = ')$' replaces the six literals,
  payloadOf() is shared by typeOf() and canonical(); unknownName() is public for
  the rules. Error messages unchanged.
Tests unchanged apart from a class name in DynamicContentMapTest.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/Compose/Placeholders.php

@@ -40,7 +40,7 @@ final class Placeholders
 
     /**
      * @param array<string, array<string, string>>|null $dynamicContent Divi's dynamic content
-     *        options for VariableRef (ruling S22); null = VariableRef::DEFAULT_DYNAMIC_CONTENT.
+     *        options for VariableRef (ruling S22); null = DynamicContentRules::DEFAULT_DYNAMIC_CONTENT.
      */
     public function __construct(private DesignSystem $design, ?array $dynamicContent = null)
     {
@@ -209,7 +209,7 @@ final class Placeholders
 
     private function field(string $name): string
     {
-        if (preg_match(VariableRef::META_KEY, $name) !== 1) {
+        if (preg_match(DynamicContentRules::META_KEY, $name) !== 1) {
             throw new InvalidArgumentException(sprintf(
                 'Ungültiger Feldschlüssel "%s" (erlaubt: Buchstaben, Ziffern, "_" und "-", nicht mit "_" am Anfang).',
                 $name