Add multilingual language and systems learning tracks

tl/rust-practice-20261101/lesson-3/main.rs

@@ -0,0 +1,8 @@
+fn remaining(total: u32, done: u32) -> u32 { total.saturating_sub(done) }
+fn main() {
+    assert_eq!(remaining(4,1),3);
+    assert_eq!(remaining(4,4),0);
+    assert_eq!(remaining(4,5),0);
+    assert_eq!(remaining(0,0),0);
+    println!("Pumasa");
+}