feat: Add UPDATE statement support

- Add Update variant to Statement enum with table, assignments, and where_clause
- Parse UPDATE table SET col = val, ... WHERE ... statements
- Support multiple column assignments in SET clause
- Implement two-phase atomic update execution in executor
- Reuse ExpressionEvaluator for WHERE clause filtering
- Add type checking for column/value type mismatches
- Add Updated variant to ExecutionResult
- Add comprehensive unit tests for parser and executor

Closes #46

Author: sshekhar563