- Create stdlib/Std/Io.bux with extern func declarations
- Rename C shim functions to short names (PrintLine, PrintInt, etc.)
- Update all 9 examples to use import Std::Io::{PrintLine, PrintInt};
- Remove manual extern func Std_Io_* declarations from examples
- Add stdlib/io.c with PrintLine, Print, PrintInt, PrintFloat, PrintBool, ReadLine
- Support extern function declarations (functions without body)
- Generate C forward declarations for extern functions
- Map Bux type names (String, int, etc.) to C types in codegen
- Update build system to copy all stdlib files
- Hello World example works: prints 'Hello, Bux!'