v0.3.0: restructure directories

- src/        ← compiler/selfhost/  (canonical Bux compiler)
- bootstrap/  ← compiler/bootstrap/ (Nim bootstrap)
- lib/        ← library/std/        (standard library)
- rt/         ← library/runtime/    (C runtime)
- tests/      ← compiler/tests/     (unit tests)
- Remove _selfhost/ (built into build/selfhost/ now)
- Update all path references (Makefile, cli.nim, cli.bux, docs)
- Bump version to 0.3.0

rt/io.c

@@ -0,0 +1,54 @@
+/* Bux Standard Library - I/O functions */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+/* PrintLine - print string with newline */
+void PrintLine(const char* s) {
+    if (s != NULL) {
+        puts(s);
+    } else {
+        puts("");
+    }
+}
+
+/* Print - print string without newline */
+void Print(const char* s) {
+    if (s != NULL) {
+        printf("%s", s);
+    }
+}
+
+/* PrintInt - print integer */
+void PrintInt(int n) {
+    printf("%d", n);
+}
+
+/* PrintInt64 - print 64-bit integer */
+void PrintInt64(int64_t n) {
+    printf("%lld", (long long)n);
+}
+
+/* PrintFloat - print float */
+void PrintFloat(double f) {
+    printf("%g", f);
+}
+
+/* PrintBool - print boolean */
+void PrintBool(int b) {
+    printf("%s", b ? "true" : "false");
+}
+
+/* ReadLine - read line from stdin (simplified) */
+const char* 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 "";
+}