Add parser support for direct field mutation via `obj.field = expr` syntax, enabling mutable object state after creation. Extend the OOP test suite with dedicated mutation tests (`oop_mutation_test.rc`) and advanced examples (`oop_advanced.rc`) covering BankAccount, Player, and Rectangle classes. Update `oop_working.rc` with mutation test cases and revise TUTORIAL.md documentation to describe field mutation, setter methods, and complete OOP examples with mutation. Add Makefile targets for running the new mutation and advanced tests.
