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>

vendor/winit/src/platform/wayland.rs

@@ -28,6 +28,36 @@ pub(crate) fn decoration_touches_ignored() -> bool {
         .is_some_and(|until| std::time::Instant::now() < until)
 }
 
+/// ink-notes patch: compositor timestamps (ms) of the touch events not yet taken, in event order.
+static TOUCH_TIMES: std::sync::Mutex<
+    std::collections::VecDeque<(u64, crate::event::TouchPhase, u32)>,
+> = std::sync::Mutex::new(std::collections::VecDeque::new());
+
+/// ink-notes patch: remembers the compositor time of a touch event that is about to be sent.
+pub(crate) fn push_touch_time(id: u64, phase: crate::event::TouchPhase, time: u32) {
+    let mut times = TOUCH_TIMES.lock().unwrap_or_else(|e| e.into_inner());
+    // Nobody takes them (e.g. an app that never asks): keep only the latest
+    if times.len() >= 256 {
+        times.pop_front();
+    }
+    times.push_back((id, phase, time));
+}
+
+/// ink-notes patch: compositor time in milliseconds (arbitrary origin, wraps around) of the
+/// touch event with this id and phase (`Started`, `Moved` or `Ended`; `Cancelled` has none),
+/// for the event currently being handled. Call it once for every such `WindowEvent::Touch`.
+/// `None` on X11 or if the time is unknown.
+pub fn take_touch_time(id: u64, phase: crate::event::TouchPhase) -> Option<u32> {
+    let mut times = TOUCH_TIMES.lock().unwrap_or_else(|e| e.into_inner());
+    // Older entries that were never taken are skipped
+    while let Some((i, p, time)) = times.pop_front() {
+        if i == id && p == phase {
+            return Some(time);
+        }
+    }
+    None
+}
+
 /// Additional methods on [`EventLoopWindowTarget`] that are specific to Wayland.
 pub trait EventLoopWindowTargetExtWayland {
     /// True if the [`EventLoopWindowTarget`] uses Wayland.