feat: add loading animation example

Add a busy-wait sleep on a monotonic clock and re-export String,
format!, and Duration from the library. The example animates a
progress bar with a spinner and logs each step in a Vec<String>.

Co-Authored-By: qwen (lmstudio/qwen3.8-27b@q3_k_xl): wrote example

runtime/main.rs

@@ -1,11 +1,55 @@
 #![no_std]
 #![no_main]
 
-fn print(message: &str) {
-    let _ = rslibc::io::write_str(message);
+use core::time::Duration;
+
+use rslibc::{format, print, println, vec, String, Vec};
+
+const FRAMES: [&str; 4] = ["|", "/", "-", "\\"];
+const WIDTH: usize = 20;
+const STEPS: usize = 10;
+
+fn bar(done: usize) -> String {
+    let mut out = String::new();
+    for i in 0..WIDTH {
+        out.push(if i < done { '#' } else { '-' });
+    }
+    out
+}
+
+fn load(step: usize) -> rslibc::Result<u32, &'static str> {
+    // Pretend to do work and return a checksum-like value.
+    let mut total = 0u32;
+    for value in vec![1, 2, 3, 4, 5] {
+        total = total.wrapping_add(value * step as u32);
+    }
+    Ok(total)
 }
 
 #[rslibc_macros::main]
 async fn main() {
-    print("Hello\n");
+    println!("rslibc loading example");
+    let mut log: Vec<String> = Vec::new();
+
+    for step in 0..STEPS {
+        match load(step) {
+            Ok(value) => {
+                let frame = FRAMES[step % FRAMES.len()];
+                let percent = 100 * (step + 1) / STEPS;
+                print!(
+                    "\r  [{}] {:>3}% {}",
+                    bar((step + 1) * WIDTH / STEPS),
+                    percent,
+                    frame
+                );
+                log.push(format!("step {step} -> {value}"));
+            }
+            Err(message) => println!("error: {message}"),
+        }
+
+        rslibc::runtime::sleep(Duration::from_millis(60));
+    }
+
+    println!("\r  [{}] 100% done", bar(WIDTH));
+    println!("log = {log:?}");
 }