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

tests/Unit/Integrations/Divi/Pages/DynamicContentMapTest.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace LIT\MCP\Tests\Unit\Integrations\Divi\Pages;
 
 use InvalidArgumentException;
+use LIT\MCP\Integrations\Divi\Compose\DynamicContentRules;
 use LIT\MCP\Integrations\Divi\Compose\Placeholders;
 use LIT\MCP\Integrations\Divi\Compose\VariableRef;
 use LIT\MCP\Integrations\Divi\Pages\DynamicContentMap;
@@ -50,7 +51,7 @@ final class DynamicContentMapTest extends TestCase
 
         foreach (Placeholders::POST as $option) {
             self::assertArrayHasKey($option, $map, $option);
-            foreach (array_keys(VariableRef::DEFAULT_DYNAMIC_CONTENT[$option]) as $setting) {
+            foreach (array_keys(DynamicContentRules::DEFAULT_DYNAMIC_CONTENT[$option]) as $setting) {
                 self::assertArrayHasKey($setting, $map[$option], "$option.$setting");
             }
         }