nmc(PF): pin mainnet_genesis_header() 80-byte seed from synced namecoind (#262)

Retire the deferred mainnet genesis-header seed. Fields pinned against a
live SYNCED mainnet namecoind (getblockhash 0 / getblockheader <hash>
false; .140 PID 5860, blocks=830010, IBD=false; 2026-06-20):
  version=1 time=1303000001 bits=0x1c007fff nonce=0xa21ea192
  merkleroot=41c62dbd..cb0d
SHA256d(header) == 000000000062b72c..c770 (mainnet genesis hash),
verified locally and by the new MainnetHeaderRederivesPinnedHash KAT.

The KAT asserts against the external literal hash (daemon truth), not
params.genesis_hash, so it holds regardless of merge order with the
mainnet genesis_hash pin (#261). Fenced to src/impl/nmc/; rides the
allowlisted nmc_auxpow_wire_test exe, no build.yml touch.

Co-authored-by: frstrtr <frstrtr@users.noreply.github.com>

Author: frstrtr

src/impl/nmc/coin/header_chain.hpp

@@ -624,6 +624,27 @@ struct NMCChainParams {
         g.m_nonce     = 384568319;
         return g;
     }
+
+    /// Namecoin MAINNET genesis header (the 80-byte parent the HeaderChain seeds
+    /// at height 0). Fields PINNED against a live SYNCED mainnet namecoind
+    /// (getblockhash 0 / getblockheader <hash> false, .140 PID 5860,
+    /// blocks=830010 IBD=false, 2026-06-20):
+    ///   version=1  time=1303000001  bits=0x1c007fff  nonce=0xa21ea192
+    ///   merkleroot=41c62dbd9068c89a449525e3cd5ac61b20ece28c3c38b3f35b2161f0e6d3cb0d
+    /// SHA256d(header) == 000000000062b72c..c770 == mainnet genesis_hash
+    /// (verified locally + by the MainnetHeaderRederivesPinnedHash KAT). This
+    /// retires the deferred mainnet_genesis_header() seed; the expected hash is
+    /// external daemon truth, not self-derived from the header fields.
+    static BlockHeaderType mainnet_genesis_header() {
+        BlockHeaderType g;
+        g.m_previous_block.SetNull();
+        g.m_version = 1;
+        g.m_merkle_root.SetHex("41c62dbd9068c89a449525e3cd5ac61b20ece28c3c38b3f35b2161f0e6d3cb0d");
+        g.m_timestamp = 1303000001;
+        g.m_bits      = 0x1c007fff;
+        g.m_nonce     = 0xa21ea192;  // 2719621522
+        return g;
+    }
 };
 
 // ─── Index Entry ─────────────────────────────────────────────────────────────

src/impl/nmc/test/auxpow_wire_test.cpp

@@ -284,6 +284,36 @@ TEST(NmcP1Genesis, TestnetHeaderRederivesPinnedHash)
     EXPECT_EQ(params.aux_chain_id, 1);  // Namecoin nAuxpowChainId = 0x0001
 }
 
+// ---------------------------------------------------------------------------
+// MAINNET genesis pin KAT (P1 PF, retires the deferred mainnet seed).
+// NMCChainParams::mainnet_genesis_header() is the 80-byte parent the embedded
+// HeaderChain seeds at height 0. Its SHA256d MUST reproduce the Namecoin
+// mainnet genesis hash pinned against a live SYNCED mainnet namecoind
+// (getblockhash 0 / getblockheader <hash> false; .140 PID 5860, blocks=830010,
+// IBD=false; 2026-06-20). The expected hash is asserted as an external literal,
+// NOT params.genesis_hash, so the guard holds regardless of merge order with
+// the mainnet genesis_hash pin (#261). A wrong field (endianness, bits, nonce,
+// merkleroot) makes the re-derived hash diverge from daemon truth.
+// ---------------------------------------------------------------------------
+TEST(NmcP1Genesis, MainnetHeaderRederivesPinnedHash)
+{
+    const auto g = nmc::coin::NMCChainParams::mainnet_genesis_header();
+
+    EXPECT_TRUE(g.m_previous_block.IsNull());
+    EXPECT_EQ(g.m_version, 1);
+    EXPECT_EQ(g.m_timestamp, 1303000001u);
+    EXPECT_EQ(g.m_bits, 0x1c007fffu);
+    EXPECT_EQ(g.m_nonce, 0xa21ea192u);
+
+    uint256 expect;
+    expect.SetHex("000000000062b72c5e2ceb45fbc8587e807c155b0da735e6483dfba2f0a9c770");
+    EXPECT_EQ(nmc::coin::block_hash(g), expect);
+
+    // Mainnet and testnet genesis headers must NOT collide (network isolation).
+    EXPECT_NE(nmc::coin::block_hash(g),
+              nmc::coin::block_hash(nmc::coin::NMCChainParams::testnet_genesis_header()));
+}
+
 // ---------------------------------------------------------------------------
 // P2P network-magic pin KAT (P1 PE 2b-ii). NMCChainParams::p2p_magic is the
 // 4-byte frame prefix the embedded won-block broadcaster (PE) puts on every