Align Lucas-Lehmer GPU helpers with windowed pow2 guidance

PerfectNumbers.Core/Gpu/GpuUInt128.cs

@@ -1020,18 +1020,8 @@ public struct GpuUInt128 : IComparable<GpuUInt128>, IEquatable<GpuUInt128>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static ulong ShiftLeftByNativeChunk(ulong value, ulong modulus)
     {
-        // TODO: Collapse this eight-step shift ladder into the ProcessEightBitWindows helper once it lands so
-        // we reuse the precomputed window residues instead of emitting `% modulus` after every shift.
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-        value = (value << 1) % modulus;
-
-        return value;
+        // Multiply by 2^NativeModuloChunkBits in one pass so the native-modulo path avoids eight `%` reductions.
+        return MulMod64(value, 1UL << NativeModuloChunkBits, modulus);
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

PerfectNumbers.Core/Gpu/MersenneNumberLucasLehmerGpuTester.cs

@@ -112,7 +112,7 @@ public class MersenneNumberLucasLehmerGpuTester
 
         var gpu = GpuContextPool.RentPreferred(preferCpu: false);
         var accelerator = gpu.Accelerator;
-        var kernel = GetBatchKernel(accelerator); // TODO: Switch this batch path to the ProcessEightBitWindows residue kernel once Lucas–Lehmer integrates the benchmarked windowed pow2 helper for small exponents.
+        var kernel = GetBatchKernel(accelerator); // Batch kernel already relies on the Mersenne squaring reducer; windowed pow2 helpers do not apply here.
 
         var expBuffer = accelerator.Allocate1D<ulong>(count);
         ulong[] expArray = ArrayPool<ulong>.Shared.Rent(count);
@@ -479,8 +479,7 @@ public class MersenneNumberLucasLehmerGpuTester
         {
             if ((exponent & 1UL) != 0UL)
             {
-                // TODO: Route these powmods through the ProcessEightBitWindows helper once it lands for GPU
-                // host-side fallbacks so the Lucas–Lehmer setup reuses the benchmarked windowed ladder.
+                // The base varies per primitive-root candidate, so pow2-specific ladders do not apply.
                 result = MulMod(result, value, modulus);
             }