Ensure GPU residue copies are synchronized and cover high divisor sets

PerfectNumbers.Core.Tests/Gpu/PrimeTesterGpuTests.cs

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+using System.Threading;
+using FluentAssertions;
+using PerfectNumbers.Core;
+using PerfectNumbers.Core.Gpu;
+using Xunit;
+
+namespace PerfectNumbers.Core.Tests.Gpu;
+
+[Trait("Category", "Fast")]
+public class PrimeTesterGpuTests
+{
+    [Fact]
+    public void IsPrimeGpu_accepts_known_primes()
+    {
+        GpuContextPool.ForceCpu = false;
+        try
+        {
+            var tester = new PrimeTester();
+            ulong[] primes = [5UL, 31UL, 61UL, 89UL, 107UL, 127UL];
+            foreach (ulong prime in primes)
+            {
+                tester.IsPrimeGpu(prime, CancellationToken.None).Should().BeTrue();
+            }
+        }
+        finally
+        {
+            GpuContextPool.ForceCpu = false;
+            GpuContextPool.DisposeAll();
+        }
+    }
+
+    [Fact]
+    public void IsPrimeGpu_rejects_composites()
+    {
+        GpuContextPool.ForceCpu = false;
+        try
+        {
+            var tester = new PrimeTester();
+            ulong[] composites = [21UL, 25UL, 27UL, 35UL, 55UL];
+            foreach (ulong composite in composites)
+            {
+                tester.IsPrimeGpu(composite, CancellationToken.None).Should().BeFalse();
+            }
+        }
+        finally
+        {
+            GpuContextPool.ForceCpu = false;
+            GpuContextPool.DisposeAll();
+        }
+    }
+
+}