feat(database): add SQL, Graph, and Raft Replication modules
- SQL store with SQLite-compatible subset (sqlparser 0.43)
- CREATE TABLE, INSERT, SELECT, UPDATE, DELETE
- WHERE clauses, ORDER BY, LIMIT
- Aggregates (COUNT, SUM, AVG, MIN, MAX)
- UNIQUE and NOT NULL constraints
- BTreeMap-based indexes
- Graph store for relationship-based queries
- Nodes with labels and properties
- Edges with types and weights
- BFS/DFS traversal
- Dijkstra shortest path
- Cypher-like query parser (MATCH, CREATE, DELETE, SET)
- Raft consensus replication for high availability
- Leader election with randomized timeouts
- Log replication with AppendEntries RPC
- Snapshot management for log compaction
- Cluster configuration and joint consensus
- Full RPC message serialization
All 159 tests pass.
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