fix: measure the swipe speed with the compositor's touch event times (Issue #28)

Pia's log shows the lift arriving 200-300 ms after the last move on every
swipe, so the release looked like a resting finger and nothing glided. The
vendored winit now keeps the Wayland timestamp of each touch event
(take_touch_time), and the fling measures with those times instead of when
the app got around to handling the events. The diagnostics log both gaps and
any event handling that blocks the app for more than 40 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/viewport.rs

@@ -389,6 +389,14 @@ impl Fling {
         velocity
     }
 
+    /// The glide continues from `now` (after [`Self::release`] with the time the finger was
+    /// lifted, which may lie a little in the past).
+    pub fn restart_clock(&mut self, now: std::time::Instant) {
+        if self.active {
+            self.last = Some(now);
+        }
+    }
+
     /// Hält an (neue Berührung, Mausklick, Mausrad).
     pub fn stop(&mut self) {
         *self = Self::default();
@@ -430,6 +438,25 @@ mod tests {
         PageStack::new(vec![A4; count], current)
     }
 
+    #[test]
+    fn test_fling_measures_with_event_times_handled_late() {
+        use std::time::{Duration, Instant};
+        let t0 = Instant::now();
+        let ms = |n: u64| t0 + Duration::from_millis(n);
+        let mut fling = Fling::default();
+        // The finger moved until 100 ms and was lifted at 105 ms; the app only got to the
+        // lift 300 ms later, so the glide starts from there
+        for i in 0..=10 {
+            fling.track(Vec2::new(0.0, -30.0), ms(i * 10));
+        }
+        let velocity = fling.release(ms(105));
+        assert!(velocity.y < -2000.0, "{velocity:?}");
+        assert!(fling.is_active());
+        fling.restart_clock(ms(405));
+        let first = fling.step(ms(421)).unwrap();
+        assert!(first.y < -20.0 && first.y > -60.0, "{first:?}");
+    }
+
     #[test]
     fn test_fling_glides_and_slows_down() {
         use std::time::{Duration, Instant};