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()`.
Introduce TOKEN_NULL to the token enum and register it as a prefix parse rule that emits CODE_NULL. Implement the null() compiler function to emit the null constant. Extend readName() to recognize the "null" keyword and map it to TOKEN_NULL. Add test files verifying null output, and that null, true, and false cannot be used as variable names.
