test(dash-g1): injectable block-version seam + byte-parity golden KAT

Make build_embedded_workdata version an injectable trailing-default param
(default 0x20000000, the BIP9 no-signaling baseline) so every existing
caller is byte-for-byte unchanged. Add G1GoldenVersionSeamEchoesInjectedVersion:
the seam echoes a pinned version exactly, the default preserves the prior
hardcoded projection, and a non-default version drifts ONLY m_version.

SAFE-ADDITIVE, fenced to embedded_gbt.hpp + test_dash_embedded_gbt.cpp,
non-consensus (default value unchanged). Companion to the curtime seam (#622).

Author: frstrtr

src/impl/dash/coin/embedded_gbt.hpp

@@ -62,7 +62,13 @@ inline DashWorkData build_embedded_workdata(
     uint32_t bits_for_next,
     uint32_t mtp_at_tip,
     uint8_t  address_version,
-    uint8_t  address_p2sh_version)
+    uint8_t  address_p2sh_version,
+    // Seam: injectable block version. Defaults to 0x20000000 (BIP9 "no
+    // signaling" baseline) so every existing caller is byte-for-byte
+    // unchanged (SAFE-ADDITIVE); the G1 golden KAT pins it, and a real
+    // BIP9-deployment-aware value can later be threaded in without touching
+    // the default header projection.
+    uint32_t version = 0x20000000u)
 {
     DashWorkData w;
     w.m_height          = prev_height + 1;
@@ -74,7 +80,7 @@ inline DashWorkData build_embedded_workdata(
     // GBT returns MTP+1 as mintime so miners don't accidentally
     // produce stale-time blocks.
     w.m_mintime         = mtp_at_tip + 1;
-    w.m_version         = 0x20000000;           // TODO: real BIP9 bits
+    w.m_version         = version;              // seam: default 0x20000000 (BIP9 baseline)
 
     // Subsidy + tx selection.
     int64_t reward = compute_dash_block_reward_post_v20(w.m_height);

test/test_dash_embedded_gbt.cpp

@@ -408,3 +408,63 @@ TEST(DashEmbeddedGbt, GbtXcheckMatchesIdenticalDetectsFieldDrift) {
     rpc_count.m_packed_payments.pop_back();
     EXPECT_FALSE(gbt_xcheck(w, rpc_count));
 }
+
+// ════════════════════════════════════════════════════════════════════════
+// G1 golden: injectable block-version seam.
+//
+// Companion to the curtime seam. build_embedded_workdata hardcoded
+// m_version = 0x20000000 (BIP9 "no signaling" baseline). With the trailing
+// defaulted `version` param, a fixed-input call yields a byte-for-byte
+// reproducible version projection, and a real BIP9-deployment-aware value
+// can later be threaded through without disturbing the default.
+//
+// Oracle discipline (non-circular): the default is asserted against the BIP9
+// baseline constant 0x20000000, and the injected path is asserted to echo the
+// pinned input exactly — the seam performs no version arithmetic of its own.
+// ════════════════════════════════════════════════════════════════════════
+
+TEST(DashEmbeddedGbt, G1GoldenVersionSeamEchoesInjectedVersion) {
+    UTXOViewCache utxo(nullptr);
+    uint256 prev = mint_hash(88);
+    utxo.add_coin(Outpoint(prev, 0), Coin(100'000, {}, /*height=*/1, /*cb=*/false));
+    Mempool mp;
+    mp.set_utxo(&utxo);
+    auto tx = make_spend(prev, 0, 90'000, /*salt=*/9);   // fee = 10'000
+    ASSERT_TRUE(mp.add_tx(tx));
+
+    auto payout   = p2pkh_script(0x55);
+    auto mnstates = single_mn(payout);
+
+    uint256 prev_hash = raw256(0xD4);
+    const uint32_t bits           = 0x1b104be3u;   // pinned oracle GBT nBits
+    const uint32_t mtp            = 1'700'000'000u;
+    const uint32_t PINNED_VERSION = 0x20000004u;   // BIP9 baseline + bit 2 set
+
+    // Injected version is echoed byte-for-byte — the seam adds no arithmetic.
+    auto injected = build_embedded_workdata(
+        H - 1, prev_hash, mnstates, mp,
+        bits, mtp, DASH_PUBKEY_VER, DASH_P2SH_VER, PINNED_VERSION);
+    EXPECT_EQ(injected.m_version, PINNED_VERSION);
+
+    // Deterministic: same fixed inputs -> identical version projection.
+    auto injected2 = build_embedded_workdata(
+        H - 1, prev_hash, mnstates, mp,
+        bits, mtp, DASH_PUBKEY_VER, DASH_P2SH_VER, PINNED_VERSION);
+    EXPECT_EQ(injected.m_version, injected2.m_version);
+
+    // Default arg preserves the prior hardcoded projection: omitting version
+    // still yields the BIP9 "no signaling" baseline, so no caller is changed.
+    auto baseline = build_embedded_workdata(
+        H - 1, prev_hash, mnstates, mp,
+        bits, mtp, DASH_PUBKEY_VER, DASH_P2SH_VER);
+    EXPECT_EQ(baseline.m_version, 0x20000000u);
+
+    // The version seam is orthogonal to every other header field: injecting a
+    // non-default version changes ONLY m_version, nothing else drifts.
+    EXPECT_EQ(injected.m_height,         baseline.m_height);
+    EXPECT_EQ(injected.m_previous_block, baseline.m_previous_block);
+    EXPECT_EQ(injected.m_bits,           baseline.m_bits);
+    EXPECT_EQ(injected.m_mintime,        baseline.m_mintime);
+    EXPECT_EQ(injected.m_coinbase_value, baseline.m_coinbase_value);
+    EXPECT_EQ(injected.m_payment_amount, baseline.m_payment_amount);
+}