Split MulHigh extensions into CPU and GPU variants

EvenPerfectBitScanner.Benchmarks/Mul64Benchmarks.cs

@@ -89,7 +89,7 @@ public class Mul64Benchmarks
     {
         ulong leftLow = (ulong)left;
         ulong rightLow = (ulong)right;
-        return ((UInt128)(leftLow * (ulong)(right >> 64) + leftLow.MulHigh(rightLow)) << 64) | (UInt128)(leftLow * rightLow);
+        return ((UInt128)(leftLow * (ulong)(right >> 64) + leftLow.MulHighCpu(rightLow)) << 64) | (UInt128)(leftLow * rightLow);
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

EvenPerfectBitScanner.Benchmarks/MulHighBenchmarks.cs

@@ -3,6 +3,7 @@ using System.Runtime.CompilerServices;
 using BenchmarkDotNet.Attributes;
 using BenchmarkDotNet.Jobs;
 using PerfectNumbers.Core;
+using PerfectNumbers.Core.Gpu;
 
 namespace EvenPerfectBitScanner.Benchmarks;
 
@@ -36,10 +37,7 @@ public class MulHighBenchmarks
     [Benchmark(Baseline = true)]
     public ulong CurrentImplementation()
     {
-        // TODO: Replace ULongExtensions.MulHigh with the UInt128-based helper on CPU paths; it was
-        // 11–14× faster in the latest benchmarks while the GPU-specific variant remains available
-        // separately.
-        return ULongExtensions.MulHigh(Input.X, Input.Y);
+        return ULongExtensions.MulHighCpu(Input.X, Input.Y);
     }
 
     /// <summary>
@@ -107,7 +105,9 @@ public class MulHighBenchmarks
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static ulong MulHighWithGpuUInt128(ulong x, ulong y)
     {
-        return ULongExtensions.MulHighGpuCompatible(x, y);
+        GpuUInt128 product = new(x);
+        GpuUInt128.Mul64(ref product, 0UL, y);
+        return product.High;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

EvenPerfectBitScanner.Benchmarks/MulMod64Benchmarks.cs

@@ -240,7 +240,7 @@ public class MulMod64Benchmarks
         ulong left = a % modulus;
         ulong right = b % modulus;
         ulong low = left * right;
-        ulong high = left.MulHigh(right);
+        ulong high = left.MulHighCpu(right);
         UInt128 product = ((UInt128)high << 64) | low;
         return (ulong)(product % modulus);
     }