Task 2: Query Execution - Implement DDL, DML, DQL with join support
- SmManager: open_db, close_db, drop_table implementations
- SeqScanExecutor: full scan with condition evaluation
- ProjectionExecutor: column projection
- NestedLoopJoinExecutor: nested loop join with condition filtering
- UpdateExecutor: record update with SET clauses and index maintenance
- DeleteExecutor: record deletion with index cleanup
- IndexScanExecutor: fallback sequential scan implementation
- Analyze: table existence checks, UpdateStmt SET clause parsing, column validation
- Planner: fix cross-join table ordering