Split CPU and GPU exponent steppers

PerfectNumbers.Core/Cpu/MersenneCpuDivisorScanSession.cs

@@ -42,7 +42,7 @@ internal sealed class MersenneCpuDivisorScanSession : IMersenneNumberDivisorByDi
 
         // Keep these remainder steppers in place so future updates continue reusing the previously computed residues.
         // They are critical for avoiding repeated full Montgomery exponentiation work when scanning divisors.
-        var exponentStepper = new ExponentRemainderStepper(cachedData);
+        var exponentStepper = new ExponentRemainderStepperCpu(cachedData);
 
         var cycleStepper = new CycleRemainderStepper(divisorCycle);
 

PerfectNumbers.Core/Cpu/MersenneNumberDivisorByDivisorCpuTester.cs

@@ -196,7 +196,7 @@ public sealed class MersenneNumberDivisorByDivisorCpuTester : IMersenneNumberDiv
                 // }
                 // else
                 {
-                    if (!MersenneDivisorCycles.TryCalculateCycleLengthForExponent(
+                    if (!MersenneDivisorCycles.TryCalculateCycleLengthForExponentCpu(
                             candidate,
                             prime,
                             divisorData,