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/BlockTree/NodeIndex.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi\BlockTree;
 
+defined('ABSPATH') || exit;
+
 /**
  * Path IDs of a Document (spec §5.3): s0, s0.r1, s0.r1.c0.m2. One letter per role –
  * s section, r row / row-inner, c column / column-inner, m module (incl. group and child