perf(audience-sample): improve scroll performance in sample app (SDK-314)

Add UsageHints.GroupTransform to both scroll view content containers so
the renderer translates each subtree on the GPU rather than re-compositing
every element on each scroll frame. Cap the log at 200 rows so the visual
tree stays bounded as events accumulate.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nattb8

examples/audience/Assets/SampleApp/Scripts/AudienceSample.UI.cs

@@ -30,6 +30,7 @@ private static readonly (string TabId, string PanelId)[] Tabs =
 
         private const int CollapseThreshold = 240;
         private const int StatusPollIntervalMs = 500;
+        private const int MaxLogRows = 200;
         private const float NarrowBreakpointPx = 1024f;
         private const float PhoneBreakpointPx  = 480f;
 
@@ -236,6 +237,11 @@ private void BindElements()
             // scroller never engages.
             _logView.contentContainer.style.flexShrink = 0;
 
+            // Tell the renderer the log subtree moves as a unit when scrolled.
+            // Without this hint Unity re-composites every row on each scroll
+            // frame; with it the subtree is translated on the GPU instead.
+            _logView.contentContainer.usageHints = UsageHints.GroupTransform;
+
             _logCount = Require<Label>("log-count");
         }
 
@@ -416,6 +422,7 @@ void Update()
                 if (float.IsNaN(needed) || needed <= 0f) return;
                 pageScroll.contentContainer.style.minHeight = needed;
             }
+            pageScroll.contentContainer.usageHints = UsageHints.GroupTransform;
             controls.RegisterCallback<GeometryChangedEvent>(_ => Update());
             logCol.RegisterCallback<GeometryChangedEvent>(_ => Update());
             pageScroll.contentContainer.schedule.Execute(Update).StartingIn(0);
@@ -759,6 +766,12 @@ private void AppendLog(string label, string? body, LogLevel level, LogSource sou
 
             var row = BuildLogRow(new LogEntry(DateTime.Now, label, body, level, source));
             _logView.Add(row);
+
+            // Keep the visual tree bounded so scroll performance doesn't degrade
+            // as the log grows. Remove the oldest row when the cap is exceeded.
+            while (_logView.contentContainer.childCount > MaxLogRows)
+                _logView.contentContainer.RemoveAt(0);
+
             _logCount.text = _logView.contentContainer.childCount.ToString(CultureInfo.InvariantCulture);
 
             // contentContainer.flexShrink = 0 (set in BindElements) makes the