* fix: don't allow go-merkledag to reorder loaded links

Ref: https://github.com/filecoin-project/boost/issues/673#issuecomment-1202312003

Ideally we can remove go-merkledag entirely from here because most (all?) of
the deal-making infrastructure uses go-codec-dagpb + go-ipld-prime traversals
and post-decode link-reordering is a subtle difference that impacts CAR
ordering for non-spec-compliant DAG-PB blocks.

* test: add test for CarOffsetWriter dag traverse order