feat: add async I/O, double type, file I/O, and break/continue to RC interpreter

Implement multi-threaded async file and socket operations with pthread-based worker pool, add double floating-point type with conversion and arithmetic functions, introduce file I/O system with fopen/fclose/fread/fwrite/fgets/fputs, and add break/continue control flow statements. Update Makefile with new test and example targets, extend README and TUTORIAL.md with comprehensive documentation and usage examples, and include async_demo.rc example and async_io_test.rc test suite.

tests/async_io_test.rc

@@ -0,0 +1,44 @@
+int main() {
+    printf("=== Async I/O Tests ===\n");
+
+    printf("Test 1: Async file write\n");
+    int f = fopen("async_test.txt", "w");
+    if (f == 0) {
+        printf("ERROR: Could not open file\n");
+        return 1;
+    }
+    int write_handle = async_fwrite(f, "Hello from async I/O!", 21);
+    printf("Write started, handle: %d\n", write_handle);
+    int result = async_wait(write_handle);
+    printf("Write completed, bytes written: %d\n", result);
+    fclose(f);
+    printf("PASS: Async write works\n");
+
+    printf("Test 2: Async file read\n");
+    f = fopen("async_test.txt", "r");
+    if (f == 0) {
+        printf("ERROR: Could not open file\n");
+        return 1;
+    }
+    int buffer[256];
+    int read_handle = async_fread(f, &buffer, 256);
+    printf("Read started, handle: %d\n", read_handle);
+
+    printf("Polling for completion...\n");
+    while (async_poll(read_handle) == 0) {
+        printf(".");
+    }
+    printf("\nRead complete!\n");
+
+    result = async_result(read_handle);
+    printf("Bytes read: %d\n", result);
+    fclose(f);
+    printf("PASS: Async read works\n");
+
+    printf("Test 3: Cleanup\n");
+    fremove("async_test.txt");
+    printf("PASS: File removed\n");
+
+    printf("\n=== All Async I/O Tests Completed ===\n");
+    return 0;
+}