qkc/types: add root and minor blocks

[blockchaindevsh]

Summary

Add QuarkChain-compatible root block and minor block types for the shard-side implementation.

Changes

• Add QuarkChain minor block, minor block header, and minor block metadata types and related functions.
• Add tests covering minor block serialization, hashing, metadata, and round trips.
• Add QuarkChain root block and root block header types and related functions.
• Add tests covering root block serialization, hashing, minor-header commitments, and round trips.
• Update genesis, shard, and raw database code to use the new block types.

Notes

Root blocks are represented for wire compatibility, hashing, ancestry, total-difficulty lookup, and minor-header processing. Root-chain construction and mining remain outside the shard implementation.

Testing

go test ./qkc/...
git diff --check

[qzhodl]

The PR description does not change yet

[ping-ke]

The PR description does not change yet

Updated

[qzhodl]

I re-checked the latest revision. Most of the previous review findings have been addressed, but these two issues are still outstanding:

1. NewMinorBlock still accepts fewer receipts than transactions, for example one transaction with zero receipts, allowing a non-empty transaction root to be paired with the empty receipt root. The original discussion is here:
   qkc/types: add root and minor blocks #36 (comment)

2. The default minor-block meta still leaves XShardTxCursorInfo nil and sets the x-shard gas limit to 12,000,000 instead of pyquarkchain's 6,000,000 default. The nil cursor serializes as a zero-valued cursor, so this is primarily an in-memory/default-value consistency issue rather than a wire-format incompatibility. The original discussion is here:
   qkc/types: add root and minor blocks #36 (comment)

Could we resolve these two remaining points before merging?