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/ShortformWriter.php

@@ -49,7 +49,7 @@ final class ShortformWriter
     public function apply(string $type, array $attrs, array $values, string $field): array
     {
         // Ruling S35: controls go before placeholder resolution and trim ("{\0{post:title}}").
-        $values = VariableRef::withoutControlsDeep($values);
+        $values = ControlChars::stripDeep($values);
         $fields = $this->modules === null ? ShortformMap::fields($type) : ($this->modules[$type] ?? []);
         $errors = [];
         foreach ($values as $name => $value) {
@@ -200,7 +200,7 @@ final class ShortformWriter
     {
         // Controls are stripped at the choke point before its marker check (S32a) – so a
         // NUL-split marker counts as a marker here too, and a canonical result is refused.
-        if ($alt === '' || VariableRef::contains(VariableRef::withoutControls($alt))) {
+        if ($alt === '' || VariableRef::contains(ControlChars::strip($alt))) {
             return '';
         }