Add dual SHA256D block check for pool mining mode

Pool sends block_target (full 256-bit network target) with each job.
Miner checks SHA256D(header + RandomX solution) for every hash against
the block target, enabling block detection at full hashrate instead of
only on submitted shares.

src/base/net/stratum/Client.cpp

@@ -418,6 +418,7 @@ bool xmrig::Client::parseJob(const rapidjson::Value &params, int *code)
     }
 
     job.setSigKey(Json::getString(params, "sig_key"));
+    job.setBlockTarget(Json::getString(params, "block_target"));
 
     m_job.setClientId(m_rpcId);