Add shift-first multiply helper and document benchmarks

EvenPerfectBitScanner.Benchmarks/HeuristicMultiplyShiftBenchmarks.cs

@@ -0,0 +1,53 @@
+using System.Collections.Generic;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[SimpleJob(RuntimeMoniker.Net80, launchCount: 1, warmupCount: 1, iterationCount: 5)]
+[MemoryDiagnoser]
+public class HeuristicMultiplyShiftBenchmarks
+{
+    private static readonly MultiplyShiftInput[] Inputs =
+    [
+        new(0x3FFF_FFFF_FFFF_FFFFUL, 3UL, 3, "NearOverflowShift"),
+        new(0x2000_0000_0000_0000UL, 7UL, 4, "HalfRange"),
+        new(0x1234_5678_9ABC_DEF0UL, 5UL, 5, "MixedBits"),
+        new(0x0000_0000_1000_0000UL, 11UL, 3, "Sparse"),
+    ];
+
+    [ParamsSource(nameof(GetInputs))]
+    public MultiplyShiftInput Input { get; set; }
+
+    public static IEnumerable<MultiplyShiftInput> GetInputs()
+    {
+        return Inputs;
+    }
+
+    [Benchmark(Baseline = true)]
+    public ulong SafeMultiplyShift()
+    {
+        return HeuristicArithmetic.MultiplyShiftRight(Input.Value, Input.Multiplier, Input.Shift);
+    }
+
+    [Benchmark]
+    public ulong ShiftFirstMultiplyShift()
+    {
+        return HeuristicArithmetic.MultiplyShiftRightShiftFirst(Input.Value, Input.Multiplier, Input.Shift);
+    }
+
+    [Benchmark]
+    public ulong NaiveMultiplyShift()
+    {
+        return (Input.Value * Input.Multiplier) >> Input.Shift;
+    }
+
+    public readonly record struct MultiplyShiftInput(ulong Value, ulong Multiplier, int Shift, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}