Adds a new btck_BlockHeader type and associated functions to create, access, and validate block headers. Block headers will have their own type (btck_BlockHeader) that can be created from raw data, copied, and queried for all the standard header fields (hash, prev hash, timestamp, bits, version, nonce). We can also extract headers from full blocks or block tree entries.

The first commit here refactors BlockValidationState to use Handle/View pattern so external code can own them, which is required for the header processing in the API.

New Block Header API

• btck_BlockHeader type: Opaque handle for block headers

• Header methods:

  • btck_block_header_create(): Create header from 80-byte serialized data

  • btck_block_header_copy(): Copy block headers

  • btck_block_header_destroy(): Destroy header object

  • btck_block_header_get_hash(): Calculate block hash

  • btck_block_header_get_prev_hash(): Get previous block hash

  • btck_block_header_get_timestamp(): Get block timestamp

  • btck_block_header_get_bits(): Get difficulty target (compact format)

  • btck_block_header_get_version(): Get block version

  • btck_block_header_get_nonce(): Get proof-of-work nonce

  • btck_block_get_header(): Extract header from a full block

  • btck_block_tree_entry_get_block_header(): Get header associated with a block tree entry

• Header Processing Methods:

  • btck_chainstate_manager_process_block_header(): Validates and processes a block header without requiring the full block. This performs proof-of-work verification, timestamp validation, and updates the internal chain state.
  • btck_chainstate_manager_get_best_entry(): Returns the block tree entry with the most cumulative proof-of-work.

Why btck_chainstate_manager_get_best_entry() is included alongside header validation? Just as we have logic to get the tip for block validation (so you can request more blocks extending your best from your peers), we need the equivalent for header validation. To make header validation worthwhile, knowing what the best current header is seems useful—it tells you what headers to request next from peers.

Testing

Added tests in test_kernel.cpp that cover creating headers from raw data, extracting all header fields, and processing headers through the chainstate manager.

CC @TheCharlatan