feat(hooks): add dice() and random() hook APIs for consensus entropy

Port the Hook API surface from the tt-rng branch, adapted to use our
commit-reveal consensus entropy (ltCONSENSUS_ENTROPY / sfDigest).

Hook APIs:
- dice(sides): returns random int [0, sides) from consensus entropy
- random(write_ptr, write_len): fills buffer with 1-512 random bytes

Internal fairRng() derives per-execution entropy by hashing: ledger
seq + tx ID + hook hash + account + chain position + execution phase
+ consensus entropy + incrementing call counter. This ensures each
call within a single hook execution returns different values.

Quality gate: fairRng returns empty (TOO_LITTLE_ENTROPY) if fewer
than 5 validators contributed, preventing weak entropy from being
consumed by hooks.

Also adds sfEntropyCount and sfLedgerSequence to the consensus
entropy SLE and pseudo-tx, enabling the freshness and quality
checks needed by the Hook API.

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

@@ -245,6 +245,8 @@ Change::applyConsensusEntropy()
         sle = std::make_shared<SLE>(keylet::consensusEntropy());
 
     sle->setFieldH256(sfDigest, entropy);
+    sle->setFieldU16(sfEntropyCount, ctx_.tx.getFieldU16(sfEntropyCount));
+    sle->setFieldU32(sfLedgerSequence, view().info().seq);
     // Note: sfPreviousTxnID and sfPreviousTxnLgrSeq are set automatically
     // by ApplyStateTable::threadItem() because isThreadedType() returns true
     // for ledger entries that have sfPreviousTxnID in their format.