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/ParserTest.php

@@ -94,6 +94,16 @@ final class ParserTest extends TestCase
         self::assertSame([], $broken->children()[0]->attrs());
     }
 
+    public function testMarksListLikeTopLevelAttributesOpaque(): void
+    {
+        $content = '<!-- wp:divi/text {"0":"a","1":"b"} /-->';
+        $document = (new Parser())->parse($content);
+
+        self::assertTrue($document->children()[0]->isOpaque());
+        self::assertSame([], $document->children()[0]->attrs());
+        self::assertSame($content, Serializer::document($document));
+    }
+
     public function testKeepsEmptyAndListLikeObjectsAsObjects(): void
     {
         $document = (new Parser())->parse($this->fixture('synthetic-mixed.html'));