Test batch Mersenne residue computation

PerfectNumbers.Core.Tests/Gpu/GpuUInt128Pow2Minus1ModTests.cs

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+using FluentAssertions;
+using PerfectNumbers.Core.Gpu;
+using Xunit;
+
+namespace PerfectNumbers.Core.Tests;
+
+[Trait("Category", "Fast")]
+public class GpuUInt128Pow2Minus1ModTests
+{
+    [Fact]
+    public void Pow2Minus1Mod_matches_pow2mod_minus_one()
+    {
+        ulong exponent = 23UL;
+        GpuUInt128 modulus = new(0UL, 97UL);
+        var pow = GpuUInt128.Pow2Mod(exponent, modulus);
+        var expected = pow.IsZero ? modulus.Sub(GpuUInt128.One) : pow.Sub(GpuUInt128.One);
+        var actual = GpuUInt128.Pow2Minus1Mod(exponent, modulus);
+        actual.Should().Be(expected);
+    }
+
+    [Fact]
+    public void Pow2Minus1ModBatch_matches_individual()
+    {
+        ulong[] exponents = { 1UL, 2UL, 3UL, 4UL, 5UL, 6UL, 7UL, 8UL };
+        Span<GpuUInt128> results = stackalloc GpuUInt128[exponents.Length];
+        GpuUInt128 modulus = new(0UL, 97UL);
+        GpuUInt128.Pow2Minus1ModBatch(modulus, exponents, results);
+        for (int i = 0; i < exponents.Length; i++)
+        {
+            var single = GpuUInt128.Pow2Minus1Mod(exponents[i], modulus);
+            results[i].Should().Be(single);
+        }
+    }
+}