feat: Divi BlockTree – byte-exact parser/serializer with round-trip fixtures

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/BlockTree/MalformedMarkup.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\BlockTree;
+
+use RuntimeException;
+
+/**
+ * post_content is no balanced block markup (closer without opener, unclosed block, PCRE
+ * failure). The Divi tools then refuse to read or write the entry – nothing is guessed.
+ */
+final class MalformedMarkup extends RuntimeException
+{
+}