since Kubo v0.12.0, blocks are stored by multihash, so identical
data with different CIDs (e.g., CIDv0 vs CIDv1) is stored once.
dag stat now reflects actual storage by using multihash-based
deduplication instead of CID-based.
updated help text to clarify deduplication behavior and note that
dag export uses CID-based keying and may include duplicates.
added regression test for multihash deduplication.