Fixing miner issues

- Switch from full-block to header-only (140-byte) RandomX input
- Add 32-byte SoloNonce system for solo mining mode
- Compute proper difficulty target from compact bits field
- Add SHA256D dual-hash PoW check in CpuWorker for solo mining
- Raise RandomX dataset/scratchpad limits to 4GB/4MB
- Use standard RandomX share filtering in pool (stratum) mode

src/net/JobResult.h

@@ -79,6 +79,22 @@ public:
     {
     }
 
+    // Constructor for solo/DragonX mining with full 32-byte nonce
+    inline JobResult(const Job &job, const uint8_t *nonce32, const uint8_t *result) :
+        algorithm(job.algorithm()),
+        index(job.index()),
+        clientId(job.clientId()),
+        jobId(job.id()),
+        backend(job.backend()),
+        nonce(0),
+        diff(job.diff()),
+        m_isSoloResult(true)
+    {
+        memcpy(m_result, result, sizeof(m_result));
+        memcpy(m_nonceBytes, nonce32, 32);
+        m_nonceSize = 32;
+    }
+
     inline const uint8_t *result() const     { return m_result; }
     inline uint64_t actualDiff() const       { return Job::toDiff(reinterpret_cast<const uint64_t*>(m_result)[3]); }
     inline uint8_t *result()                 { return m_result; }
@@ -87,6 +103,12 @@ public:
 
     inline const uint8_t *minerSignature() const { return m_hasMinerSignature ? m_minerSignature : nullptr; }
 
+    // Solo nonce support
+    inline bool isSoloResult() const         { return m_isSoloResult; }
+    inline const uint8_t *soloNonce() const  { return m_nonceBytes; }
+    inline const uint8_t *nonceBytes() const { return m_nonceBytes; }
+    inline size_t nonceSize() const          { return m_nonceSize; }
+
     const Algorithm algorithm;
     const uint8_t index;
     const String clientId;
@@ -102,6 +124,9 @@ private:
 
     uint8_t m_minerSignature[64] = { 0 };
     bool m_hasMinerSignature = false;
+    uint8_t m_nonceBytes[32] = { 0 };
+    size_t m_nonceSize = 4;
+    bool m_isSoloResult = false;
 };