test(audience-sample): pin frame rate to 1 on Linux for llvmpipe fill rate

examples/audience/Assets/SampleApp/Tests/Runtime/LinuxRenderSuppression.cs

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+#nullable enable
+
+#if UNITY_STANDALONE_LINUX
+using NUnit.Framework;
+using UnityEngine;
+
+namespace Immutable.Audience.Samples.SampleApp.Tests
+{
+    // Linux-only test suppression: clamp the player's frame rate to 1 fps
+    // and disable vsync. The SampleApp PlayMode tests assert on UI Toolkit
+    // visual-element state, which is layout-driven, not paint-driven.
+    // Painting fewer frames between coroutine yields removes llvmpipe
+    // fragment-fill cost without changing what tests observe.
+    //
+    // Scope: every test in the Tests assembly when the build target is
+    // StandaloneLinux64. UNITY_STANDALONE_LINUX is defined by Unity for
+    // PlayMode runs invoked with -testPlatform StandaloneLinux64.
+    [SetUpFixture]
+    public sealed class LinuxRenderSuppression
+    {
+        private int _priorTargetFrameRate;
+        private int _priorVSyncCount;
+
+        [OneTimeSetUp]
+        public void Suppress()
+        {
+            _priorTargetFrameRate = Application.targetFrameRate;
+            _priorVSyncCount = QualitySettings.vSyncCount;
+
+            Application.targetFrameRate = 1;
+            QualitySettings.vSyncCount = 0;
+        }
+
+        [OneTimeTearDown]
+        public void Restore()
+        {
+            Application.targetFrameRate = _priorTargetFrameRate;
+            QualitySettings.vSyncCount = _priorVSyncCount;
+        }
+    }
+}
+#endif