Define and test bitcoind header json serializer

[dsbaars]

Summary

chain/json/header exposes the wire (native) and electrumx_tag serializers but not a
bitcoind_tag variant. The block serializer (src/chain/json/block.cpp) already carries the
full bitcoind family (bitcoind_tag / bitcoind_hashed_tag / bitcoind_verbose_tag /
bitcoind_embedded_tag) and inlines the header fields into the block's bitcoind encoding, but
there is no standalone bitcoind header serializer. Consumers that need a Core-format header JSON
must hand-roll it.

Proposed

Mirror the existing electrum header serializer (#1822):

• include/bitcoin/system/chain/json/header.hpp: add DECLARE_JSON_TAGGED_INVOKE(bitcoind_tag, header).
• src/chain/json/header.cpp: add DEFINE_JSON_FROM_TAGGED(bitcoind_tag, header).
• test/chain/json/header.cpp: add header__json__bitcoind__expected.

Fields mirror the header portion of DEFINE_JSON_FROM_TAGGED(bitcoind_tag, block):
hash, version, versionHex, merkleroot, time, nonce, bits, difficulty.

Chain-context properties (confirmations, height, mediantime, previousblockhash,
nextblockhash) stay a consumer responsibility, consistent with the block bitcoind serializer
(which documents them as chain, not block, properties).

Precedent

• Define and test electrum header json serializer. #1822 "Define and test electrum header json serializer" added DEFINE_JSON_FROM_TAGGED(electrumx_tag, header). This is the exact analog for bitcoind_tag.
• src/chain/json/block.cpp DEFINE_JSON_FROM_TAGGED(bitcoind_tag, block) is the reference for field formatting (versionHex / bits as big-endian base16, difficulty as the computed double).

Consumer

libbitcoin/libbitcoin-server#795 hand-rolls this as header_to_bitcoind in
protocol_bitcoind_rpc_json.cpp. Once this lands it collapses to value_from(bitcoind(*header))
in both getblockheader and the REST /rest/headers handler, and the protocol-layer helper is
removed. Raised from review feedback on that PR.

Testing

Exact-string assertion via json::value_from(bitcoind(instance)), as in #1822. Reusing the
block bitcoind test instance (header{ 42, null_hash, one_hash, 43, 44, 45 }) yields
"versionHex":"0000002a", "bits":"0000002c", "difficulty":1.0279680609929991E73.