nmc: build-side merged-mining commitment (PC-prep, build/verify symmetry) - #408
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…try) Add nmc::coin::build_mm_commitment(), the build-side inverse of scan_mm_commitment(): emit the canonical merged-mining commitment payload a parent (BTC) coinbase scriptSig must carry for an embedded NMC aux chain. Byte layout matches the scanner the verifier already runs bit-for-bit: MM_HEADER_MAGIC(4) | chain_merkle_root REVERSED(32) | size_le32 | nonce_le32, size = 2^chain_height, slot = aux_expected_index(nonce, chain_id, height). PB-independent (no NameOp/mempool dependency); overlaps PD dual-target. NMC-local per the coin fence (std + core types only); btc tree READ-ONLY. +3 KATs in nmc_auxpow_merkle_test (already allowlisted, no build.yml change): round-trip emit->scan MATCH at the expected slot; explicit byte-layout pin; tampered root/slot -> MISMATCH. 77->80 tests, all green.
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What
Adds
nmc::coin::build_mm_commitment()— the build-side inverse of the existingscan_mm_commitment()verifier. Emits the canonical merged-mining commitment payload a parent (BTC) coinbase scriptSig must carry so an embedded NMC aux chain is provable.Byte layout is bit-for-bit what the scanner walks:
MM_HEADER_MAGIC(4) | chain_merkle_root REVERSED(32) | size_le32 | nonce_le32, size = 2^chain_height, slot =aux_expected_index(nonce, chain_id, height).Why
Closes the build/verify symmetry gap: the verify side (slot derivation, scanner, aux_merkle_root) already exists; the build side that PD wires into the dual-target parent coinbase did not. PB-independent (no NameOp/mempool dependency) — cut off master per slice-base rule, NOT stacked on #402.
Tests
+3 KATs in
nmc_auxpow_merkle_test(already allowlisted — no build.yml change, no #137 NOT_BUILT trap): round-trip emit->scan MATCH at the expected slot; explicit byte-layout pin; tampered root/slot -> MISMATCH. 77->80, all green locally.Fence
src/impl/nmc/only (header_chain.hpp + auxpow_merkle_test.cpp). NMC-local, std + core types only; btc tree consumed READ-ONLY; no core/shared/other-coin edits.