test(ui): the fake sound card waits to pull until the starting frame is checked

Under load its thread pulled samples before the test read the clock, so
'from the first frame' saw frame 1 and a_started_preview_follows_its_sound flaked.

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

crates/ui-egui/tests/it/ui_audio_scrub.rs

@@ -553,8 +553,10 @@ fn a_device_that_fails_to_start_plays_the_picture() {
 }
 
 /// An output that plays: a thread takes the feed's samples in real time, as a sound card would.
+/// It holds off pulling until `.1` is set, so a test can read the clock before any sample is
+/// taken however loaded the machine is (the thread otherwise races the test's first check).
 #[derive(Clone, Default)]
-struct Playing(Arc<Mutex<bool>>);
+struct Playing(Arc<Mutex<bool>>, Arc<std::sync::atomic::AtomicBool>);
 
 impl AudioDevice for Playing {
     fn sample_rate(&self) -> u32 {
@@ -562,9 +564,14 @@ impl AudioDevice for Playing {
     }
     fn start(&mut self, feed: Arc<AudioFeed>) -> Result<(), String> {
         let stop = self.0.clone();
+        let go = self.1.clone();
         std::thread::spawn(move || {
             let mut buf = vec![0.0f32; 480 * 2];
             while !*stop.lock().unwrap() {
+                if !go.load(std::sync::atomic::Ordering::Acquire) {
+                    std::thread::sleep(std::time::Duration::from_millis(1));
+                    continue;
+                }
                 feed.pull(&mut buf);
                 std::thread::sleep(std::time::Duration::from_millis(10));
             }
@@ -590,6 +597,7 @@ fn a_started_preview_follows_its_sound() {
     h.run_steps(2);
     assert!(until_started(&mut h) && h.state().audio.is_some(), "the sound started");
     assert_eq!(audio_frame(&h, cid), Some(0), "from the first frame");
+    device.1.store(true, std::sync::atomic::Ordering::Release);
     let deadline = std::time::Instant::now() + std::time::Duration::from_secs(30);
     while frame(&h, cid) < 5 && std::time::Instant::now() < deadline && h.state().audio.is_some() {
         step(&mut h);