refactor: migrate examples to import Std::Io instead of raw extern func

- 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

examples/hello.bux

@@ -1,7 +1,7 @@
 // Hello World - Basic Bux program
-extern func Std_Io_PrintLine(s: String);
+import Std::Io::{PrintLine, PrintInt};
 
 func Main() -> int {
-    Std_Io_PrintLine("Hello, Bux!");
+    PrintLine("Hello, Bux!");
     return 0;
 }