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

stdlib/io.c

@@ -5,7 +5,7 @@
 #include <string.h>
 
 /* PrintLine - print string with newline */
-void Std_Io_PrintLine(const char* s) {
+void PrintLine(const char* s) {
     if (s != NULL) {
         puts(s);
     } else {
@@ -14,29 +14,29 @@ void Std_Io_PrintLine(const char* s) {
 }
 
 /* Print - print string without newline */
-void Std_Io_Print(const char* s) {
+void Print(const char* s) {
     if (s != NULL) {
         printf("%s", s);
     }
 }
 
 /* PrintInt - print integer */
-void Std_Io_PrintInt(int64_t n) {
+void PrintInt(int64_t n) {
     printf("%lld", (long long)n);
 }
 
 /* PrintFloat - print float */
-void Std_Io_PrintFloat(double f) {
+void PrintFloat(double f) {
     printf("%g", f);
 }
 
 /* PrintBool - print boolean */
-void Std_Io_PrintBool(int b) {
+void PrintBool(int b) {
     printf("%s", b ? "true" : "false");
 }
 
 /* ReadLine - read line from stdin (simplified) */
-const char* Std_Io_ReadLine(void) {
+const char* ReadLine(void) {
     static char buffer[1024];
     if (fgets(buffer, sizeof(buffer), stdin) != NULL) {
         size_t len = strlen(buffer);