Return scalar MulMod results and benchmark native reducers

EvenPerfectBitScanner.Benchmarks/GpuPow2ModBenchmarks.cs

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+using System.Collections.Generic;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core.Gpu;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[SimpleJob(RuntimeMoniker.Net80)]
+[MemoryDiagnoser]
+public class GpuPow2ModBenchmarks
+{
+    private static readonly Pow2ModInput[] Inputs = new Pow2ModInput[]
+    {
+        new(63UL, new GpuUInt128(0UL, 97UL), "SmallExponentSmallModulus"),
+        new(1_048_575UL, new GpuUInt128(0UL, 0xFFFF_FFFBUL), "MediumExponentPrimeModulus"),
+        new(6_710_886_400UL, new GpuUInt128(0x0000_0000_0000_0001UL, 0xFFFF_FFFF_FFFF_FFC3UL), "LargeExponentHighWordModulus"),
+        new(ulong.MaxValue, new GpuUInt128(0xFFFF_FFFF_FFFF_FFFBUL, 0xFFFF_FFFF_FFFF_FFC5UL), "FullWidthExponentFullWidthModulus"),
+    };
+
+    [ParamsSource(nameof(GetInputs))]
+    public Pow2ModInput Input { get; set; }
+
+    public static IEnumerable<Pow2ModInput> GetInputs() => Inputs;
+
+    [Benchmark(Baseline = true)]
+    public GpuUInt128 ProcessEightBitWindows()
+    {
+        return GpuUInt128.Pow2Mod(Input.Exponent, Input.Modulus);
+    }
+
+    [Benchmark]
+    public GpuUInt128 ProcessSingleBits()
+    {
+        return Pow2ModSingleBit(Input.Exponent, Input.Modulus);
+    }
+
+    private static GpuUInt128 Pow2ModSingleBit(ulong exponent, in GpuUInt128 modulus)
+    {
+        if (modulus.IsZero || modulus == GpuUInt128.One)
+        {
+            return new GpuUInt128();
+        }
+
+        GpuUInt128 result = new(1UL);
+        GpuUInt128 baseVal = new(2UL);
+
+        ulong e = exponent;
+        while (e != 0UL)
+        {
+            if ((e & 1UL) != 0UL)
+            {
+                result.MulMod(baseVal, modulus);
+            }
+
+            baseVal.MulMod(baseVal, modulus);
+            e >>= 1;
+        }
+
+        return result;
+    }
+
+    public readonly record struct Pow2ModInput(ulong Exponent, GpuUInt128 Modulus, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}