Replace the global `io` singleton object with a static `IO` class, moving the `write` native from the instance to the metaclass. Update all benchmark, example, and test files to use `IO.write()` instead of `io.write()`.
Implement list subscript setter (`[ ]=`) native method in wren_core.c, enabling assignment to list elements via bracket syntax. Extract duplicate parameter count validation logic into a shared `validateNumParameters` function in wren_compiler.c, replacing inline checks in both `parameterList` and `methodCall`. Add comprehensive test suite covering basic assignment, return value semantics, negative indices, and error cases for out-of-range and excessive arguments.
