nmc(P1f-a): canonical CAuxPow Serialize + persist auxpow blob behind has_auxpow

[frstrtr]

Sub-leg (a) of NMC P1f, per integrator GO (port the canonical Namecoin CAuxPow wire layout, fenced to src/impl/nmc/). Stacked on #201.

What

• nmc::coin::AuxPow gains Serialize/Unserialize: a byte-faithful port of Namecoin CAuxPow / classic Bitcoin auxpow field order — CMerkleTx coinbase leg (parent_coinbase TX_NO_WITNESS, parent_block_hash, parent_coinbase_branch, parent_coinbase_index) -> chain-merkle leg (chain_merkle_branch, chain_merkle_index) -> parent_header (80-byte CPureBlockHeader). Parent coinbase is witness-stripped to match the txid the parent tx-merkle tree commits (same stance as parent_coinbase_txid()).
• IndexEntryDiskV1 now persists the blob behind has_auxpow (write iff ==1; restore in to_entry()); the nmc(P1f): HeaderChain LevelDB persistence (h/tip/height) + 6 KATs (52->58) #201 deferred flag-only leg is removed.
• +2 KATs: codec full round-trip + still-VALID through check_proof; IndexEntryDiskV1 blob round-trip (+ proofless => nullopt). nmc(P1f): HeaderChain LevelDB persistence (h/tip/height) + 6 KATs (52->58) #201 deferred-blob test updated to assert restoration. 64->66 green.

Fence

Touches src/impl/nmc/ only — no bitcoin_family/ or src/core/. Target nmc_auxpow_merkle_test already in build.yml allowlist (no CI change).

Consensus note / merge handling

Adds a consensus serialization shape — per integrator this does NOT auto-merge: CI-verify, then operator tap. Immediate follow-up: ground-truth wire-vector assertion serializing a captured Namecoin testnet auxpow and asserting the daemon bytes (KAT currently self-round-trips + verifies).