feat: add stdlib IO and extern function support

- 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!'

stdlib/io.c

@@ -0,0 +1,49 @@
+/* Bux Standard Library - I/O functions */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+/* PrintLine - print string with newline */
+void Std_Io_PrintLine(const char* s) {
+    if (s != NULL) {
+        puts(s);
+    } else {
+        puts("");
+    }
+}
+
+/* Print - print string without newline */
+void Std_Io_Print(const char* s) {
+    if (s != NULL) {
+        printf("%s", s);
+    }
+}
+
+/* PrintInt - print integer */
+void Std_Io_PrintInt(int64_t n) {
+    printf("%lld", (long long)n);
+}
+
+/* PrintFloat - print float */
+void Std_Io_PrintFloat(double f) {
+    printf("%g", f);
+}
+
+/* PrintBool - print boolean */
+void Std_Io_PrintBool(int b) {
+    printf("%s", b ? "true" : "false");
+}
+
+/* ReadLine - read line from stdin (simplified) */
+const char* Std_Io_ReadLine(void) {
+    static char buffer[1024];
+    if (fgets(buffer, sizeof(buffer), stdin) != NULL) {
+        size_t len = strlen(buffer);
+        if (len > 0 && buffer[len-1] == '\n') {
+            buffer[len-1] = '\0';
+        }
+        return buffer;
+    }
+    return "";
+}