nmc: PD-prep golden fixture — pin dual-target MM-commitment byte layout

[frstrtr]

What

Golden-fixture KATs that pin PD acceptance target ahead of the build code. They hand-compute the 44-byte merged-mining marker from the Namecoin/Bitcoin auxpow.cpp reference layout and assert it as an explicit byte literal:

[ fa be 6d 6d ]  4-byte MM magic
[ 32 bytes    ]  chain-merkle root, REVERSED (big-endian display order)
[ 4 bytes LE  ]  tree size = 2^chain_height
[ 4 bytes LE  ]  nonce

#408-independent by construction

Does NOT import or invoke build_mm_commitment (PR #408 PD surface). The literal is an independent oracle PD output must reproduce byte-for-byte — not a mirror of the build side. Zero stack-rot risk; cut off master, not stacked on #408.

Binds to the already-merged verify SSOT:

• scan_mm_commitment returns MATCH on the live single-slot posture (h=0) and a height-2 multi-slot case, both bare and after scriptSig framing;
• a wrong-slot presentation is shown to MISMATCH (slot field is load-bearing);
• aux_expected_index cross-checks the deterministic slot;
• field offsets [0,4)/[4,36)/[36,40)/[40,44) each checked against independent literals.

Fence / CI

• src/impl/nmc/ test target only; additive (+3 KATs, suite 77->80 local green).
• Rides already-allowlisted nmc_auxpow_merkle_test — no build.yml change, no NOT_BUILT trap, no consensus-value change.
• Qualifies for auto-merge under the fenced-additive rule.