bch(kat): coinbase DGB<->BCH pairing slice 1 -- gated-segwit-OFF predicate pin

Pins the one real DGB<->BCH coinbase divergence: BCH gates the SegWit
commitment vout on is_segwit_activated(version), which the p2poolBCH oracle
derives from getattr(net,"SEGWIT_ACTIVATION_VERSION",0) -> 0 -> false for
every share version. Asserts the predicate is OFF across v17..v36 (and the
sentinel stays disabled), so a regression re-introducing a non-zero
SEGWIT_ACTIVATION_VERSION (the old stray-17 port -> sharechain fork) fails CI.
Wired into BCH_ABLA_TESTS (pure, header-only over share_types.hpp).
Byte-vector half (full coinbase vs oracle ground-truth) is the follow slice.

Separate BCH-lane PR per per-coin isolation; #171 stays DGB-tree only.
p2pool-merged-v36 surface: none.

Author: frstrtr

src/impl/bch/test/CMakeLists.txt

@@ -18,6 +18,7 @@ if(BUILD_TESTING)
         block_download_test       # M5: windowed headers-first block download (pure)
         genesis_conformance_test  # M5: genesis-hash pin vs BCHN v29.0.0 chainparams (pure)
         abla_growth_soak_test     # M5: dynamic ABLA budget-GROWTH proof above 32 MB floor (pure)
+        coinbase_kat_segwit_predicate_test # KAT: DGB<->BCH coinbase pairing -- gated-segwit-OFF predicate pin (pure)
     )
     foreach(t IN LISTS BCH_ABLA_TESTS)
         add_executable(bch_${t} ${t}.cpp)

src/impl/bch/test/coinbase_kat_segwit_predicate_test.cpp

@@ -0,0 +1,64 @@
+// ---------------------------------------------------------------------------
+// bch coinbase KAT -- gated-segwit predicate pin (DGB<->BCH pairing, slice 1).
+//
+// The DGB<->BCH coinbase-byte KAT pairing (integrator PR #171 DGB side; this is
+// the SEPARATE BCH-lane PR per the per-coin isolation invariant) has ONE real
+// divergence between the two SHA256d coinbases: the SegWit-commitment output
+// the builder gates on `is_segwit_activated(version)` at serialize time.
+//
+//   - DGB (SegWit active): the gate is TRUE for share versions >= its
+//     SEGWIT_ACTIVATION_VERSION, so the coinbase carries the witness-commitment
+//     vout (OP_RETURN 0x6a24aa21a9ed || commitment).
+//   - BCH (SegWit rejected at the 2017 fork): the oracle p2poolBCH derives the
+//     gate from getattr(net,"SEGWIT_ACTIVATION_VERSION",0) (data.py:63-66); the
+//     bitcoincash[_testnet].py nets define NO such attribute -> default 0 ->
+//     the gate is FALSE for EVERY version -> NO witness-commitment vout is ever
+//     emitted, and segwit_data is never present in a BCH share.
+//
+// This slice pins exactly that predicate: across the full v17..v36 share-version
+// range, bch::is_segwit_activated MUST be false, matching the oracle's
+// default-0 guard. A regression that re-introduces a non-zero
+// SEGWIT_ACTIVATION_VERSION (the old stray-17 BTC/LTC port that forced
+// segwit_data on for v17..v35 -> sharechain fork; see share_types.hpp) fails
+// here. The byte-vector half of the KAT (full coinbase vs oracle ground-truth
+// from p2poolBCH util/pack) is the follow sub-slice.
+//
+// p2pool-merged-v36 surface: NONE (pins a conformance invariant, adds no wire
+// field). per-coin isolation: src/impl/bch/ only. Header-only over
+// share_types.hpp -- no peer, socket, or coin lib.
+// ---------------------------------------------------------------------------
+
+#include <cassert>
+#include <iostream>
+
+#include "../share_types.hpp"
+
+int main()
+{
+    // The sentinel itself must stay disabled (== 0), reproducing the oracle
+    // getattr(net,"SEGWIT_ACTIVATION_VERSION",0) default for the BCH nets.
+    assert(bch::SEGWIT_ACTIVATION_VERSION == 0
+           && "BCH must keep SEGWIT_ACTIVATION_VERSION disabled (oracle default 0)");
+
+    // The predicate must be FALSE for every share version BCH ships -- the full
+    // v17..v36 span and a few sentinels around the boundaries. There is no
+    // version at which a BCH coinbase emits the witness-commitment vout.
+    for (uint64_t ver = 0; ver <= 64; ++ver)
+    {
+        assert(!bch::is_segwit_activated(ver)
+               && "BCH gated-segwit predicate must be OFF at every share version");
+    }
+
+    // Spot-check the versions that actually matter on the sharechain: the
+    // legacy floor (17), the field-order transition versions, and v36.
+    for (uint64_t ver : {uint64_t(17), uint64_t(33), uint64_t(34),
+                         uint64_t(35), uint64_t(36)})
+    {
+        assert(!bch::is_segwit_activated(ver));
+    }
+
+    std::cout << "bch coinbase-KAT segwit-predicate pin: OK "
+              << "(is_segwit_activated == false for all versions; "
+              << "no witness-commitment vout on any BCH coinbase)\n";
+    return 0;
+}