fix(security): no direct HTTP access to plugin PHP files (audit L2, cheap part)

Every file under src/ exits outside WordPress (defined('ABSPATH') ||
exit; after the use block, so a direct request no longer prints a fatal
error with the server path); divi-convert-fixture.php gets the WP_CLI
guard the other integration scripts have. A unit test keeps both true;
PSR1 side-effects warning excluded for the guard.

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

@@ -7,6 +7,8 @@ namespace LIT\MCP\Integrations\Divi\Compose;
 use InvalidArgumentException;
 use LIT\MCP\Integrations\Divi\Design\DesignSystem;
 
+defined('ABSPATH') || exit;
+
 /**
  * Translates the placeholders of spec §5.4 into Divi's $variable(…)$ syntax:
  * {{color:primary|<Name>|gcid-…}}, {{var:<Name>|gvid-…}}, {{font:heading|body}},