chain: add WATTX block-header variant (enables WTX/WATTx header sync + swaps)

[nucash-mining]

What

Adds a WATTX chain variant to the block-header reader so KDF can parse WATTx (WTX) headers. Two files, +49/-2:

• mm2_bitcoin/serialization/src/reader.rs — ChainVariant::WATTX (+ is_wattx(), "WATTX" parse).
• mm2_bitcoin/chain/src/block_header.rs — WATTX branch in BlockHeader deserialization.

Why

WATTx is a Qtum fork, so it uses Qtum's extended header (180-byte base + varbytes block signature + hashStateRoot/hashUTXORoot/prevoutStake). But WATTx additionally appends a 40-byte Gapcoin tail on every block — nShift (4) + nAdder (32) + nGapSize (4) — and the block hash is computed over that full extended header. It also sets the AuxPoW version bit on merged-mined blocks without carrying an AuxPoW payload in the header.

With the stock QTUM variant, blockchain.block.headers batches from a WATTx ElectrumX mis-parse (Transaction has no inputs-style errors as the reader runs past the header), which breaks WTX header sync and therefore WTX atomic swaps. The WATTX variant: (1) reads the Qtum fields, (2) skips the in-header AuxPoW read for WATTx, and (3) consumes the trailing 40 bytes.

Test

chain::block_header::tests::test_wattx_block_headers_batch_11 decodes a real 11-header batch (blockchain.block.headers(3121, 11) from a live WATTx ElectrumX). Passes:

test block_header::tests::test_wattx_block_headers_batch_11 ... ok

Context

Pairs with the coins-repo entry GLEECBTC/coins#1955, which sets protocol.chain_variant: WATTX for WTX and points at a live WSS ElectrumX (electrum-wtx.wattxchange.app:443). Once this variant is in the hosted kdf build and dex.gleec.com is rebuilt, WTX activates and swaps. Single self-contained commit, cherry-picked clean onto main.