nmc(P1 PE 2b-0): pin testnet genesis + chain_id=1 in NMCChainParams

Replace the TO-CONFIRM placeholders in NMCChainParams::testnet() with the
real Namecoin testnet genesis, pinned against a live namecoind on .140
(getblockhash 0 / getblockheader, 2026-06-19):
  version=1 time=1296688602 bits=0x1d07fff8 nonce=384568319
  merkleroot=4a5e1e4b..deda33b  ->  hash=00000007..367b008

Add NMCChainParams::testnet_genesis_header() (the 80-byte parent the
embedded HeaderChain seeds at height 0) and pin aux_chain_id=1
(Namecoin nAuxpowChainId=0x0001, both nets). New GenesisTestnet KAT
re-derives the pinned hash via block_hash() (external daemon truth, not
self-derived). Rides the allowlisted nmc_auxpow_wire_test exe -> no
build.yml change. Mainnet genesis stays TO-CONFIRM: pinning needs a
namecoin MAINNET node (.140 is currently on testnet, height 0).

Fence: src/impl/nmc/ only.

Author: frstrtr

src/impl/nmc/coin/header_chain.hpp

@@ -526,7 +526,7 @@ struct NMCChainParams {
     int32_t testnet_auxpow_activation_height{-1}; // TO-CONFIRM: testnet analog (unpinned sentinel)
     // aux_chain_id: the chain id this NMC instance claims in the parent's
     //   merged-mining tree. -1 = unpinned sentinel.
-    int32_t aux_chain_id{-1};               // TO-CONFIRM: Namecoin mainnet chain id (unpinned)
+    int32_t aux_chain_id{1};                // Namecoin nAuxpowChainId = 0x0001 (both nets); see chainparams.cpp
 
     int64_t difficulty_adjustment_interval() const {
         return target_timespan / target_spacing;
@@ -565,12 +565,31 @@ struct NMCChainParams {
         p.target_spacing  = TESTNET_TARGET_SPACING;
         p.allow_min_difficulty = TESTNET_ALLOW_MIN_DIFF;
         p.no_retargeting = false;
-        // TO-CONFIRM: Namecoin testnet powLimit (placeholder = BTC default).
+        // Namecoin testnet powLimit = Bitcoin default (chainparams.cpp CTestNetParams).
         p.pow_limit.SetHex("00000000ffff0000000000000000000000000000000000000000000000000000");
-        // TO-CONFIRM: Namecoin testnet genesis hash (placeholder = null).
-        p.genesis_hash.SetNull();
+        // Genesis PINNED against a live namecoind on namecoin testnet
+        // (getblockhash 0 / getblockheader, 2026-06-19). block_hash(genesis_header)
+        // re-derives this value (nmc_auxpow_wire_test GenesisTestnet KAT).
+        p.genesis_hash.SetHex("00000007199508e34a9ff81e6ec0c477a4cccff2a4767a8eee39c11db367b008");
         return p;
     }
+
+    /// Namecoin testnet genesis header (the 80-byte parent the HeaderChain seeds
+    /// at height 0). Fields PINNED against a live namecoind getblockheader on
+    /// namecoin testnet, 2026-06-19:
+    ///   version=1  time=1296688602  bits=0x1d07fff8  nonce=384568319
+    ///   merkleroot=4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b
+    /// SHA256d(header) == genesis_hash() above (verified locally + by KAT).
+    static BlockHeaderType testnet_genesis_header() {
+        BlockHeaderType g;
+        g.m_previous_block.SetNull();
+        g.m_version = 1;
+        g.m_merkle_root.SetHex("4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b");
+        g.m_timestamp = 1296688602;
+        g.m_bits      = 0x1d07fff8;
+        g.m_nonce     = 384568319;
+        return g;
+    }
 };
 
 // ─── Index Entry ─────────────────────────────────────────────────────────────

src/impl/nmc/test/auxpow_wire_test.cpp

@@ -258,4 +258,29 @@ TEST(NmcP1AuxPowWire, MainnetBlock829949RoundTrip)
     EXPECT_LT(80 + auxlen, total);
 }
 
+// ---------------------------------------------------------------------------
+// Genesis pin KAT (P1 PE 2b-0). NMCChainParams::testnet_genesis_header() is the
+// 80-byte parent the embedded HeaderChain seeds at height 0. Its SHA256d MUST
+// reproduce NMCChainParams::testnet().genesis_hash, which was pinned against a
+// live namecoind (getblockhash 0 / getblockheader) on namecoin testnet,
+// 2026-06-19. A wrong field (endianness, bits, nonce, merkleroot) makes the
+// re-derived hash diverge from the daemon-sourced value rather than be mirrored
+// -- the expected hash is external truth, not self-derived from the header.
+// chain_id is also pinned to Namecoin's nAuxpowChainId (0x0001).
+// ---------------------------------------------------------------------------
+TEST(NmcP1Genesis, TestnetHeaderRederivesPinnedHash)
+{
+    const auto params = nmc::coin::NMCChainParams::testnet();
+    const auto g      = nmc::coin::NMCChainParams::testnet_genesis_header();
+
+    EXPECT_TRUE(g.m_previous_block.IsNull());
+    EXPECT_EQ(nmc::coin::block_hash(g), params.genesis_hash);
+
+    uint256 expect;
+    expect.SetHex("00000007199508e34a9ff81e6ec0c477a4cccff2a4767a8eee39c11db367b008");
+    EXPECT_EQ(params.genesis_hash, expect);
+
+    EXPECT_EQ(params.aux_chain_id, 1);  // Namecoin nAuxpowChainId = 0x0001
+}
+
 }  // namespace