Return scalar MulMod results and benchmark native reducers

EvenPerfectBitScanner.Benchmarks/GpuUInt128AddBenchmarks.cs

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+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core.Gpu;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[SimpleJob(RuntimeMoniker.Net80)]
+[MemoryDiagnoser]
+public class GpuUInt128AddBenchmarks
+{
+    private static readonly AddInput[] Inputs = new AddInput[]
+    {
+        new(new GpuUInt128(0UL, 0UL), new GpuUInt128(0UL, 1UL), "IncrementLow"),
+        new(new GpuUInt128(0UL, ulong.MaxValue), new GpuUInt128(0UL, 1UL), "CarryIntoHigh"),
+        new(new GpuUInt128(1UL, ulong.MaxValue), new GpuUInt128(3UL, 7UL), "MixedOperands"),
+        new(new GpuUInt128(ulong.MaxValue, ulong.MaxValue), new GpuUInt128(ulong.MaxValue, ulong.MaxValue), "AllBitsSet"),
+    };
+
+    [ParamsSource(nameof(GetInputs))]
+    public AddInput Input { get; set; }
+
+    public static IEnumerable<AddInput> GetInputs() => Inputs;
+
+    [Benchmark(Baseline = true)]
+    public GpuUInt128 CarryMaterialisedInLocal()
+    {
+        GpuUInt128 value = Input.Left;
+        value.Add(Input.Right);
+        return value;
+    }
+
+    [Benchmark]
+    public GpuUInt128 CarryInExpression()
+    {
+        GpuUInt128 value = Input.Left;
+        AddInline(ref value, Input.Right);
+        return value;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static void AddInline(ref GpuUInt128 left, GpuUInt128 right)
+    {
+        ulong low = left.Low + right.Low;
+        left.High = left.High + right.High + (low < left.Low ? 1UL : 0UL);
+        left.Low = low;
+    }
+
+    public readonly record struct AddInput(GpuUInt128 Left, GpuUInt128 Right, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}