feat(consensus): add featureConsensusEntropy amendment gating

- Register ConsensusEntropy amendment (Supported::yes, DefaultNo)
- Gate entropy pseudo-tx injection behind amendment in doAccept()
- Gate preflight with temDISABLED when amendment not enabled
- Bump numFeatures 90 -> 91
- Exclude featureConsensusEntropy from default test environment to
  avoid breaking existing test transaction count assumptions

src/ripple/app/consensus/RCLConsensus.cpp

@@ -524,9 +524,12 @@ RCLConsensus::Adaptor::doAccept(
         }
     }
 
-    // Inject consensus entropy pseudo-transaction
+    // Inject consensus entropy pseudo-transaction (if amendment enabled)
     // This must happen before buildLCL so the entropy tx is in the ledger
-    injectEntropyPseudoTx(retriableTxs, prevLedger.seq() + 1);
+    if (prevLedger.ledger_->rules().enabled(featureConsensusEntropy))
+        injectEntropyPseudoTx(retriableTxs, prevLedger.seq() + 1);
+    else
+        clearRngState();
 
     auto built = buildLCL(
         prevLedger,

src/ripple/app/tx/impl/Change.cpp

@@ -98,12 +98,11 @@ Change::preflight(PreflightContext const& ctx)
 
     if (ctx.tx.getTxnType() == ttCONSENSUS_ENTROPY)
     {
-        // TODO: Add amendment gating when the feature is finalized
-        // if (!ctx.rules.enabled(featureConsensusEntropy))
-        // {
-        //     JLOG(ctx.j.warn()) << "Change: ConsensusEntropy is not enabled.";
-        //     return temDISABLED;
-        // }
+        if (!ctx.rules.enabled(featureConsensusEntropy))
+        {
+            JLOG(ctx.j.warn()) << "Change: ConsensusEntropy is not enabled.";
+            return temDISABLED;
+        }
 
         if (!ctx.tx.isFieldPresent(sfDigest))
         {

src/ripple/protocol/Feature.h

@@ -74,7 +74,7 @@ namespace detail {
 // Feature.cpp. Because it's only used to reserve storage, and determine how
 // large to make the FeatureBitset, it MAY be larger. It MUST NOT be less than
 // the actual number of amendments. A LogicError on startup will verify this.
-static constexpr std::size_t numFeatures = 90;
+static constexpr std::size_t numFeatures = 91;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated
@@ -378,6 +378,7 @@ extern uint256 const fixInvalidTxFlags;
 extern uint256 const featureExtendedHookState;
 extern uint256 const fixCronStacking;
 extern uint256 const fixHookAPI20251128;
+extern uint256 const featureConsensusEntropy;
 }  // namespace ripple
 
 #endif

src/ripple/protocol/impl/Feature.cpp

@@ -484,6 +484,7 @@ REGISTER_FIX    (fixInvalidTxFlags,             Supported::yes, VoteBehavior::De
 REGISTER_FEATURE(ExtendedHookState,             Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FIX    (fixCronStacking,               Supported::yes, VoteBehavior::DefaultYes);
 REGISTER_FIX    (fixHookAPI20251128,            Supported::yes, VoteBehavior::DefaultYes);
+REGISTER_FEATURE(ConsensusEntropy,              Supported::yes, VoteBehavior::DefaultNo);
 
 // The following amendments are obsolete, but must remain supported
 // because they could potentially get enabled.

src/test/jtx/Env.h

@@ -82,7 +82,10 @@ supported_amendments()
                 Throw<std::runtime_error>(
                     "Unknown feature: " + s + "  in supportedAmendments.");
         }
-        return FeatureBitset(feats);
+        // TODO: ConsensusEntropy injects a pseudo-tx every ledger which
+        // breaks existing test transaction count assumptions. Exclude from
+        // default test set until dedicated tests are written.
+        return FeatureBitset(feats) - featureConsensusEntropy;
     }();
     return ids;
 }