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/algebraic_enums.bux

@@ -1,6 +1,6 @@
 // Algebraic Enums - Enums with data (tagged unions)
-extern func Std_Io_PrintLine(s: String);
-extern func Std_Io_PrintInt(n: int);
+import Std::Io::{PrintLine, PrintInt};
+
 
 enum Result {
     Ok(int),
@@ -15,14 +15,14 @@ func Main() -> int {
     r2.data.Err_0 = "error message";
     
     if r1.tag == Result_Ok {
-        Std_Io_PrintLine("r1 is Ok:");
-        Std_Io_PrintInt(r1.data.Ok_0);
-        Std_Io_PrintLine("");
+        PrintLine("r1 is Ok:");
+        PrintInt(r1.data.Ok_0);
+        PrintLine("");
     }
     
     if r2.tag == Result_Err {
-        Std_Io_PrintLine("r2 is Err:");
-        Std_Io_PrintLine(r2.data.Err_0);
+        PrintLine("r2 is Err:");
+        PrintLine(r2.data.Err_0);
     }
     
     return 0;