fix: harden the Divi BlockTree against opaque/invalid writes

Final fix wave for stage 2a, load-bearing for 2b's writers:
- Block::setAttrs()/Container::insertChild()/removeChild() refuse on an opaque
  block (undecodable attrs or a global module) instead of silently corrupting it.
- Container::append() is now @internal (Parser only) and refuses on a void
  block, so a child can no longer be silently dropped from the serialized output.
- Block::create() validates the block name against namespace/block-name.
- Json::decodeObject() returns null (-> opaque) for a top-level object whose
  keys are a plain list ("0","1",...) rather than returning [] and letting the
  Serializer re-encode it as a corrupting JSON array.
- AttrMap::set() rejects empty path segments and keeps an existing stdClass
  value an object when rewrapping it (no silent object->list flip); AttrMap::get()
  is updated in step so it can still read back through such an object.
- Block::copy() deep-clones stdClass attribute values instead of sharing them.
- Json::encode()'s existing JsonException-on-invalid-UTF-8 behaviour is documented.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/Divi/BlockTree/JsonTest.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\BlockTree;
+
+use JsonException;
+use LIT\MCP\Integrations\Divi\BlockTree\Json;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class JsonTest extends TestCase
+{
+    public function testDecodesAPlainObject(): void
+    {
+        self::assertSame(['a' => 1], Json::decodeObject('{"a":1}'));
+        self::assertSame([], Json::decodeObject('{}'));
+    }
+
+    public function testRejectsNonObjectTopLevelJson(): void
+    {
+        self::assertNull(Json::decodeObject('[1,2,3]'));
+        self::assertNull(Json::decodeObject('"a string"'));
+        self::assertNull(Json::decodeObject('not json'));
+    }
+
+    /** A JSON object whose keys are "0", "1", … decodes to a PHP list – re-encoding it would
+     *  silently turn it back into a JSON array (Json.php docblock, EmptyArrayCorruptionMigration). */
+    public function testTreatsAListLikeTopLevelObjectAsOpaqueRatherThanAnArray(): void
+    {
+        self::assertNull(Json::decodeObject('{"0":"a","1":"b"}'));
+        self::assertNull(Json::decodeObject('{"0":"a"}'));
+    }
+
+    public function testDoesNotFlagAGenuineObjectWithNonSequentialNumericKeys(): void
+    {
+        self::assertSame(['0' => 'a', '2' => 'b'], Json::decodeObject('{"0":"a","2":"b"}'));
+    }
+
+    public function testEncodeThrowsOnInvalidUtf8(): void
+    {
+        $this->expectException(JsonException::class);
+
+        Json::encode("\xB1\x31");
+    }
+
+    public function testEncodeRoundtripsAnEmptyObject(): void
+    {
+        self::assertSame('{}', Json::encode(new stdClass()));
+    }
+}