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/factorial.bux

@@ -1,6 +1,6 @@
 // Factorial - Recursive function
-extern func Std_Io_PrintLine(s: String);
-extern func Std_Io_PrintInt(n: int);
+import Std::Io::{PrintLine, PrintInt};
+
 
 func Factorial(n: int) -> int {
     if n <= 1 {
@@ -10,15 +10,15 @@ func Factorial(n: int) -> int {
 }
 
 func Main() -> int {
-    Std_Io_PrintLine("Factorials:");
+    PrintLine("Factorials:");
     
     var i: int = 1;
     while i <= 10 {
         let fact: int = Factorial(i);
-        Std_Io_PrintInt(i);
-        Std_Io_PrintLine("! = ");
-        Std_Io_PrintInt(fact);
-        Std_Io_PrintLine("");
+        PrintInt(i);
+        PrintLine("! = ");
+        PrintInt(fact);
+        PrintLine("");
         i = i + 1;
     }