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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>