.dxnb decoder — the mirror image of Dextrin.Binary.Encoder.
Envelope (magic version cbor_item) checked once at the top;
everything below that is a single recursive decode_item/2, keyed
on CBOR major type first, then on tag — matching DXN.md §2.2's
table row order the same way the encoder does.
Summary
Functions
@spec decode( binary(), keyword() ) :: {:ok, term()} | {:error, Dextrin.Error.t()}