Fix RandomX validation exploit: verify nSolution contains valid RandomX hash

- Add CheckRandomXSolution() to validate RandomX PoW in nSolution field
- Add ASSETCHAINS_RANDOMX_VALIDATION activation height per chain
  (DRAGONX: 2838976, TUMIN: 1200, others: height 1)
- Add CRandomXInput serializer for deterministic RandomX hash input
- Fix CheckProofOfWork() to properly reject invalid PoW (was missing
  SMART_CHAIN_SYMBOL check, allowing bypass)
- Call CheckRandomXSolution() in hush_checkPOW and CheckBlockHeader

Without this fix, attackers could submit blocks with invalid RandomX
hashes that passed validation, as CheckProofOfWork returned early
during block loading and the nSolution field was never verified.

src/pow.h

@@ -38,6 +38,18 @@ unsigned int CalculateNextWorkRequired(arith_uint256 bnAvg,
 /** Check whether the Equihash solution in a block header is valid */
 bool CheckEquihashSolution(const CBlockHeader *pblock, const CChainParams&);
 
+/** Check whether a block header contains a valid RandomX solution */
+bool CheckRandomXSolution(const CBlockHeader *pblock, int32_t height);
+
+/** Set thread-local flag to skip RandomX validation (used by miner during TestBlockValidity) */
+void SetSkipRandomXValidation(bool skip);
+
+/** Return the RandomX key rotation interval in blocks */
+int GetRandomXInterval();
+
+/** Return the RandomX key change lag in blocks */
+int GetRandomXBlockLag();
+
 /** Check whether a block hash satisfies the proof-of-work requirement specified by nBits */
 bool CheckProofOfWork(const CBlockHeader &blkHeader, uint8_t *pubkey33, int32_t height, const Consensus::Params& params);
 CChainPower GetBlockProof(const CBlockIndex& block);