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

@@ -65,9 +65,11 @@ extern "C" fn main() {
 }
 
 fn handle_input(snake: &mut Snake) {
+    // 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),
@@ -78,6 +80,7 @@ fn handle_input(snake: &mut Snake) {
             }
         }
     }
+}
 
 fn wait_tick() {
     let start = syscall::get_time();

crates/drivers/input/virtio/src/lib.rs

@@ -129,6 +129,7 @@ fn linux_keycode_to_key(code: u16) -> Option<Key> {
 
 impl InputDevice for VirtioInputDevice {
     fn poll(&mut self) -> Option<InputEvent> {
+        loop {
             let current_used = self.used_idx();
             if current_used == self.last_used {
                 return None;
@@ -149,8 +150,8 @@ impl InputDevice for VirtioInputDevice {
                     }
                 }
             }
-
+            // Skip SYN events, repeats, and unknown keys — keep draining
-        None
+        }
     }
 
     fn has_event(&self) -> bool {