dgb(test): pin LIVE make_coin_params subsidy_func binding (#334)

The era-boundary tests trust kSubsidyFunc, a hand-written duplicate of
params.hpp p.subsidy_func. They pin the schedule math but are blind to a
regression in the actual production wiring: if make_coin_params() shipped
subsidy_func unbound (work source logs subsidy_func=UNSET, embedded
coinbasevalue silently degrades to the external-GBT-only path) or bound it
to the wrong function, every existing test would still pass off its private
copy. Add DgbCoinParams.SubsidyFuncBoundToOracleScheduleInProduction: pull
the real make_coin_params(false).subsidy_func, assert it is bound, and that
it reproduces the oracle schedule at every reward-era boundary.

Fenced test-only (src/impl/dgb/test/), no consensus-value change; extends an
existing CI target so no allowlist/drift-guard change.

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

Author: frstrtr

src/impl/dgb/test/work_source_test.cpp

@@ -18,6 +18,7 @@
 #include <impl/dgb/coin/mempool.hpp>
 #include <impl/dgb/coin/connection_coinbase.hpp>  // build_connection_coinbase_from_pplns (SSOT under test)
 #include <impl/dgb/config_coin.hpp>   // dgb::CoinParams::subsidy (oracle SSOT)
+#include <impl/dgb/params.hpp>        // dgb::make_coin_params -- the LIVE production binding
 
 #include <core/pow.hpp>                 // core::SubsidyFunc
 
@@ -460,6 +461,28 @@ TEST(DgbWorkSource, CoinbaseValueHonorsGbtVerbatim)
 }
 
 
+// Production-binding guard. The era-boundary tests above trust kSubsidyFunc, a
+// hand-written DUPLICATE of params.hpp p.subsidy_func: they pin the schedule
+// math but are blind to a regression in the ACTUAL wiring. If make_coin_params()
+// shipped subsidy_func unbound (the work source then logs subsidy_func=UNSET and
+// the embedded coinbasevalue silently falls through to the GBT-only path) or
+// bound it to the wrong function, every test above would still pass off its
+// private copy. Pin the LIVE production binding directly.
+TEST(DgbCoinParams, SubsidyFuncBoundToOracleScheduleInProduction)
+{
+    const core::CoinParams p = dgb::make_coin_params(/*testnet=*/false);
+    ASSERT_TRUE(static_cast<bool>(p.subsidy_func))
+        << "make_coin_params shipped subsidy_func UNSET -- embedded coinbasevalue "
+           "would silently degrade to the external-GBT-only path";
+    for (const auto& v : kEraBoundaries) {
+        EXPECT_EQ(p.subsidy_func(v.height), v.subsidy)
+            << "production subsidy_func diverged from oracle subsidy at " << v.era;
+        EXPECT_EQ(p.subsidy_func(v.height), kSubsidyFunc(v.height))
+            << "production binding diverged from the schedule under test at " << v.era;
+    }
+}
+
+
 // previousblockhash: emitted as GBT-conventional big-endian display hex ONLY
 // when the HeaderChain carries a real tip hash (tip_hash() accessor). With a
 // known tip block_hash, the template surfaces it MSB-limb-first; bits stays