Port smessage to RocksDB, make RocksDB a hard dep

The secure-messaging store (smsgDB) used the LevelDB API directly. Mass-
mapped to the equivalent RocksDB types: leveldb::DB/Status/WriteBatch/
Iterator/Slice/ReadOptions/WriteOptions/WriteBatch::Handler -> rocksdb::*.
The RocksDB API surface for our usage is binary-compatible — pure namespace
substitution, no semantic changes. Consumers in rpcsmessage.cpp and
qt/messagemodel.cpp updated to match.

RocksDB now becomes a hard build dependency (was optional behind
BUILD_ROCKSDB). The chain-DB rocksdb backend is consequently always
available; -chaindb=leveldb remains the default until the Phase-4
LevelDB retirement. Removed the BUILD_ROCKSDB cmake option, the
#ifdef BUILD_ROCKSDB guards in txdb*, and the runtime error path
that triggered when the flag was off.

CI updated: librocksdb-dev (Ubuntu), mingw-w64-x86_64-rocksdb (MSYS2),
and rocksdb (Homebrew) added to all build jobs.

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

src/txdb-rocksdb.cpp

@@ -2,8 +2,6 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#ifdef BUILD_ROCKSDB
-
 #include "txdb-rocksdb.h"
 
 #include <map>
@@ -698,5 +696,3 @@ bool CRocksTxDB::LoadBlockIndex()
 
     return true;
 }
-
-#endif // BUILD_ROCKSDB