fix: VirtIO keyboard input latency and direction accuracy

- Driver now drains SYN/repeat/unknown events in a loop instead of
  returning None on the first non-key event
- Snake drains all pending input events per tick, applies only the
  latest direction change (prevents stale event accumulation)

crates/apps/snake/src/main.rs

@@ -65,16 +65,19 @@ extern "C" fn main() {
 }
 
 fn handle_input(snake: &mut Snake) {
-    let _ = syscall::send(INPUT_EP, 0, 0, 0, 0);
-    let (tag, kind, _, _) = syscall::receive(INPUT_EP);
-
-    if tag == 1 {
-        match kind {
-            0 => snake.set_direction(Direction::Up),
-            1 => snake.set_direction(Direction::Down),
-            2 => snake.set_direction(Direction::Left),
-            3 => snake.set_direction(Direction::Right),
-            _ => {}
+    // Drain all pending events, apply the latest direction
+    loop {
+        let _ = syscall::send(INPUT_EP, 0, 0, 0, 0);
+        let (tag, kind, _, _) = syscall::receive(INPUT_EP);
+        if tag == 0 { break; } // NoEvent — queue drained
+        if tag == 1 {
+            match kind {
+                0 => snake.set_direction(Direction::Up),
+                1 => snake.set_direction(Direction::Down),
+                2 => snake.set_direction(Direction::Left),
+                3 => snake.set_direction(Direction::Right),
+                _ => {}
+            }
         }
     }
 }