feat(consensus): add ttCONSENSUS_ENTROPY pseudo-transaction protocol layer

Add protocol definitions for consensus-derived entropy pseudo-transaction:
- ttCONSENSUS_ENTROPY = 105 transaction type
- ltCONSENSUS_ENTROPY = 0x0058 ledger entry type
- keylet::consensusEntropy() singleton keylet (namespace 'X')
- applyConsensusEntropy() handler in Change.cpp
- Added to isPseudoTx() in STTx.cpp

The entropy value is stored in sfDigest field of the singleton ledger object.
This provides the protocol foundation for same-ledger entropy injection.

src/ripple/protocol/impl/TxFormats.cpp

@@ -490,6 +490,14 @@ TxFormats::TxFormats()
             {sfStartTime, soeOPTIONAL},
         },
         commonFields);
+
+    add(jss::ConsensusEntropy,
+        ttCONSENSUS_ENTROPY,
+        {
+            {sfLedgerSequence, soeREQUIRED},
+            {sfDigest, soeREQUIRED},  // The computed entropy value
+        },
+        commonFields);
 }
 
 TxFormats const&