[wheel] preserve grouped writes for structured objects (#3133)

* [wheel] preserve grouped writes for structured objects

* [wheel] allow cleanup-only object ids in bundle manifests

* [wheel] address structured object group review comments

* [wheel] normalize empty structured group ids

* [wheel] persist structured storage group in handles

* Restore grouped tensor fast path

* Preserve explicit ungrouped handles

* Accept transitional v2 DataProto handles