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>

src/Integrations/Divi/BlockTree/Serializer.php

@@ -8,7 +8,8 @@ namespace LIT\MCP\Integrations\Divi\BlockTree;
  * Turns a Document back into block markup. Unchanged blocks reuse their original
  * delimiters; changed or new blocks get delimiters generated exactly like WordPress'
  * get_comment_delimited_block_content() / serialize_block_attributes() do, which also
- * yields the escaping Divi expects inside $variable(…)$ payloads (" → ").
+ * yields the escaping Divi expects inside $variable(…)$ payloads: json_encode()'s own
+ * \" (an escaped quote inside a JSON string) is replaced by ", see ESCAPES below.
  */
 final class Serializer
 {