- Remove hash field from IPFSBlock struct; content hash is resolved at
 fetch time rather than stored in the block
- rawFetcher uses b.cid.hash directly (raw: CID hash == content hash)
- dagPbFileFetcher delegates to getDagPbFileHash stub (throws until
 CAR-based hash resolution is implemented)
- Simplify dagPbFileBlock restriction to b.path != null
- Remove hash from mkBlockArgs and mkBlock (dag-pb file hash will be
 derived automatically; raw hash comes from CID)
- fetchFromIpfs retains hash parameter for cid.parseHash construction
- Remove testFetchFromIpfsDagPbWithHash (explicit hash no longer supported
 for dag-pb file blocks)