C++20 modernization: nullptr, constexpr, smart pointers, thread safety, enum class

- Replace ~320 NULL occurrences with nullptr across 47 files (-184 net lines)
- static const -> constexpr for version, coin, utility constants
- Collapse 9 PushMessage overloads into 1 variadic template with fold expressions
- Convert boost::array -> std::array, boost::type_traits -> std:: equivalents
- pwalletMain, pScriptCheckQueue, pScriptCheckThreads -> unique_ptr
- mapOrphanBlocks values: raw CBlock* -> unique_ptr<CBlock>
- Fix data races: add locks to wallet registration, pindexBest reads, mempool exists
- pwalletdbEncryption: raw new/delete -> local unique_ptr, remove exit() calls
- PoS reward overflow: CBigNum intermediate for nCoinAge * nRewardCoinYear
- memset -> OPENSSL_cleanse for secure zeroing
- Fix const-cast UB in SetMerkleBranch
- Log silent catch(...) blocks instead of silently swallowing
- Enum class: GetMinFeeMode, WalletFeature
- std::string_view for 8 utility function parameters
- Range-for with structured bindings: 63 iterator loops modernized
- std::make_pair -> brace init: 35 sites
- Delegating constructors: CWallet, CBlockIndex
- Merkle tree caching, std::array for GetMedianTimePast
- CScript copy ctor -> = default, operator!= -> = default

src/script.cpp

@@ -16,7 +16,7 @@ using namespace std;
 #include "sync.h"
 #include "util.h"
 
-bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CScript scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
+bool CheckSig(const vector<unsigned char>& vchSig, const vector<unsigned char>& vchPubKey, const CScript& scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
 
 static const valtype vchFalse(0);
 static const valtype vchZero(0);
@@ -103,7 +103,7 @@ const char* GetTxnOutputType(txnouttype t)
     case TX_SCRIPTHASH: return "scripthash";
     case TX_MULTISIG: return "multisig";
     }
-    return NULL;
+    return nullptr;
 }
 
 
@@ -893,7 +893,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
                         break;
 
                     case OP_DIV:
-                        if (!BN_div(bn.get(), NULL, bn1.get(), bn2.get(), pctx))
+                        if (!BN_div(bn.get(), nullptr, bn1.get(), bn2.get(), pctx))
                             return false;
                         break;
 
@@ -1271,7 +1271,7 @@ public:
     }
 };
 
-bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CScript scriptCode,
+bool CheckSig(const vector<unsigned char>& vchSig, const vector<unsigned char>& vchPubKey, const CScript& scriptCode,
               const CTransaction& txTo, unsigned int nIn, int nHashType)
 {
     static CSignatureCache signatureCache;