Fix three CI breakages on the C++20 baseline

1. rocksdb::WriteBatch::Handler typeinfo missing on Ubuntu's librocksdb-dev.
   Both SecMsgBatchScanner (smessage.cpp) and CRocksBatchScanner
   (txdb-rocksdb.cpp) inherited from Handler to scan an active WriteBatch
   for pending writes/deletes; that subclass-based scan fails to link
   because Ubuntu's package hides the parent's typeinfo. Replaced both
   scanners with a parallel std::map<std::string, std::optional<std::string>>
   maintained alongside each WriteBatch — Put adds a value entry, Delete
   adds a nullopt entry, ScanBatch becomes an O(log n) map lookup. Same
   semantics, no Handler dependency.

2. macOS Homebrew's RocksDB 10.x removed the raw DB** overload of
   DB::Open; only std::unique_ptr<DB>* remains. txdb-rocksdb.cpp called
   the raw form, breaking the macOS build. Added the same SFINAE Open
   wrapper used in smessage.cpp (commit 4265343) that picks whichever
   overload the linked rocksdb actually has.

3. CSignal<>'s SignalState::slots member collided with Qt's `#define slots`
   to empty, stripping the member name in any TU that pulls in <QtCore>
   (e.g. moc-generated files that include util_signal.h transitively).
   Renamed to slot_map.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/smessage.cpp

@@ -450,59 +450,29 @@ bool SecMsgDB::Open(const char* pszMode)
 };
 
 
-class SecMsgBatchScanner : public rocksdb::WriteBatch::Handler
-{
-public:
-    std::string needle;
-    bool* deleted;
-    std::string* foundValue;
-    bool foundEntry;
-
-    SecMsgBatchScanner() : foundEntry(false) {}
-
-    virtual void Put(const rocksdb::Slice& key, const rocksdb::Slice& value) override
-    {
-        if (key.ToString() == needle)
-        {
-            foundEntry = true;
-            *deleted = false;
-            *foundValue = value.ToString();
-        };
-    };
-
-    virtual void Delete(const rocksdb::Slice& key) override
-    {
-        if (key.ToString() == needle)
-        {
-            foundEntry = true;
-            *deleted = true;
-        };
-    };
-};
-
 // When performing a read, if we have an active batch we need to check it first
 // before reading from the database, as the rest of the code assumes that once
-// a database transaction begins reads are consistent with it. It would be good
-// to change that assumption in future and avoid the performance hit, though in
-// practice it does not appear to be large.
+// a database transaction begins reads are consistent with it.
+//
+// Previously implemented via rocksdb::WriteBatch::Handler subclass, which fails
+// to link against Ubuntu's librocksdb-dev (typeinfo for the Handler base class
+// isn't exported there). The pendingBatch map is updated alongside every Put
+// or Delete on activeBatch and answers ScanBatch queries directly.
 bool SecMsgDB::ScanBatch(const CDataStream& key, std::string* value, bool* deleted) const
 {
     if (!activeBatch)
         return false;
-    
+
     *deleted = false;
-    SecMsgBatchScanner scanner;
-    scanner.needle = key.str();
-    scanner.deleted = deleted;
-    scanner.foundValue = value;
-    rocksdb::Status s = activeBatch->Iterate(&scanner);
-    if (!s.ok())
-    {
-        printf("SecMsgDB ScanBatch error: %s\n", s.ToString().c_str());
+    auto it = pendingBatch.find(key.str());
+    if (it == pendingBatch.end())
         return false;
-    };
-    
-    return scanner.foundEntry;
+    if (!it->second.has_value()) {
+        *deleted = true;
+        return true;
+    }
+    *value = *it->second;
+    return true;
 }
 
 bool SecMsgDB::TxnBegin()
@@ -510,6 +480,7 @@ bool SecMsgDB::TxnBegin()
     if (activeBatch)
         return true;
     activeBatch = new rocksdb::WriteBatch();
+    pendingBatch.clear();
     return true;
 };
 
@@ -523,6 +494,7 @@ bool SecMsgDB::TxnCommit()
     rocksdb::Status status = pdb->Write(writeOptions, activeBatch);
     delete activeBatch;
     activeBatch = NULL;
+    pendingBatch.clear();
     
     if (!status.ok())
     {
@@ -537,6 +509,7 @@ bool SecMsgDB::TxnAbort()
 {
     delete activeBatch;
     activeBatch = NULL;
+    pendingBatch.clear();
     return true;
 };
 
@@ -602,9 +575,10 @@ bool SecMsgDB::WritePK(CKeyID& addr, CPubKey& pubkey)
     if (activeBatch)
     {
         activeBatch->Put(ssKey.str(), ssValue.str());
+        pendingBatch[ssKey.str()] = ssValue.str();
         return true;
     };
-    
+
     rocksdb::WriteOptions writeOptions;
     writeOptions.sync = true;
     rocksdb::Status s = pdb->Put(writeOptions, ssKey.str(), ssValue.str());
@@ -613,7 +587,7 @@ bool SecMsgDB::WritePK(CKeyID& addr, CPubKey& pubkey)
         printf("SecMsgDB write failure: %s\n", s.ToString().c_str());
         return false;
     };
-    
+
     return true;
 };
 
@@ -746,9 +720,10 @@ bool SecMsgDB::WriteSmesg(unsigned char* chKey, SecMsgStored& smsgStored)
     if (activeBatch)
     {
         activeBatch->Put(ssKey.str(), ssValue.str());
+        pendingBatch[ssKey.str()] = ssValue.str();
         return true;
     };
-    
+
     rocksdb::WriteOptions writeOptions;
     writeOptions.sync = true;
     rocksdb::Status s = pdb->Put(writeOptions, ssKey.str(), ssValue.str());
@@ -757,7 +732,7 @@ bool SecMsgDB::WriteSmesg(unsigned char* chKey, SecMsgStored& smsgStored)
         printf("SecMsgDB write failed: %s\n", s.ToString().c_str());
         return false;
     };
-    
+
     return true;
 };
 
@@ -792,9 +767,10 @@ bool SecMsgDB::EraseSmesg(unsigned char* chKey)
     if (activeBatch)
     {
         activeBatch->Delete(ssKey.str());
+        pendingBatch[ssKey.str()] = std::nullopt;
         return true;
     };
-    
+
     rocksdb::WriteOptions writeOptions;
     writeOptions.sync = true;
     rocksdb::Status s = pdb->Delete(writeOptions, ssKey.str());