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/TreeEditor.php

@@ -9,6 +9,8 @@ use LIT\MCP\Integrations\Divi\BlockTree\Container;
 use LIT\MCP\Integrations\Divi\BlockTree\Document;
 use LIT\MCP\Integrations\Divi\BlockTree\NodeIndex;
 
+defined('ABSPATH') || exit;
+
 /**
  * Structural changes on a parsed page (spec §5.7): where to insert (parent / before / after,
  * optional index), move, remove, duplicate – always checked against StructureRules. Works on