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/primitives/block.h

@@ -237,6 +237,33 @@ public:
     }
 };
 
+/**
+ * Custom serializer for CBlockHeader that includes nNonce but omits nSolution,
+ * for use as deterministic input to RandomX hashing.
+ */
+class CRandomXInput : private CBlockHeader
+{
+public:
+    CRandomXInput(const CBlockHeader &header)
+    {
+        CBlockHeader::SetNull();
+        *((CBlockHeader*)this) = header;
+    }
+
+    ADD_SERIALIZE_METHODS;
+
+    template <typename Stream, typename Operation>
+    inline void SerializationOp(Stream& s, Operation ser_action) {
+        READWRITE(this->nVersion);
+        READWRITE(hashPrevBlock);
+        READWRITE(hashMerkleRoot);
+        READWRITE(hashFinalSaplingRoot);
+        READWRITE(nTime);
+        READWRITE(nBits);
+        READWRITE(nNonce);
+    }
+};
+
 
 /** Describes a place in the block chain to another node such that if the
  * other node doesn't have the same branch, it can find a recent common trunk.