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/base/net/stratum/HushClient.h

@@ -77,11 +77,11 @@ private:
     
     // Block serialization
     std::vector<uint8_t> serializeBlock(uint32_t nonce, uint64_t extraNonce) const;
-    std::vector<uint8_t> serializeHeader(uint32_t nonce, const uint8_t* solution) const;
-    std::string serializeBlockHex(uint32_t nonce, const uint8_t* solution) const;
+    std::vector<uint8_t> serializeHeader(const uint8_t* nonce32, const uint8_t* solution) const;
+    std::string serializeBlockHex(const uint8_t* nonce32, const uint8_t* solution) const;
     
     // PoW verification (SHA256D of header with solution)
-    bool checkPow(uint32_t nonce, const uint8_t* solution) const;
+    bool checkPow(const uint8_t* nonce32, const uint8_t* solution) const;
     static void sha256d(const uint8_t* data, size_t len, uint8_t* out);
 
     // Target/difficulty conversion
@@ -117,6 +117,13 @@ private:
     std::vector<uint8_t> m_headerBlob;
     size_t m_nonceOffset = 0;  // Offset of nNonce field in the block blob
 
+    // Cached binary header fields for block serialization (avoids re-parsing hex each submit)
+    uint8_t m_headerPrevHash[32] = {};
+    uint8_t m_headerMerkleRoot[32] = {};
+    uint8_t m_headerSaplingRoot[32] = {};
+    uint32_t m_headerTime = 0;
+    uint32_t m_headerBits = 0;
+
     // Request tracking
     enum RequestType { REQ_NONE, REQ_TEMPLATE, REQ_KEYHASH, REQ_SUBMIT };
     RequestType m_pendingRequest = REQ_NONE;