Use LastDigit enum across Mersenne scanners

PerfectNumbers.Core/Cpu/MersenneNumberIncrementalCpuTester.cs

@@ -5,7 +5,7 @@ public class MersenneNumberIncrementalCpuTester(GpuKernelType kernelType)
     private static readonly MersenneDivisorCycles _cycles = MersenneDivisorCycles.Shared;
     private readonly GpuKernelType _kernelType = kernelType;
 
-    public void Scan(ulong exponent, UInt128 twoP, bool lastIsSeven, UInt128 maxK, ref bool isPrime)
+    public void Scan(ulong exponent, UInt128 twoP, LastDigit lastDigit, UInt128 maxK, ref bool isPrime)
     {
         UInt128 k = UInt128.One;
         UInt128 one = k;

PerfectNumbers.Core/Cpu/MersenneNumberOrderCpuTester.cs

@@ -4,7 +4,7 @@ public class MersenneNumberOrderCpuTester(GpuKernelType kernelType)
 {
 	private readonly GpuKernelType _kernelType = kernelType;
 
-	public void Scan(ulong exponent, UInt128 twoP, bool lastIsSeven, UInt128 maxK, ref bool isPrime)
+	public void Scan(ulong exponent, UInt128 twoP, LastDigit lastDigit, UInt128 maxK, ref bool isPrime)
 	{
 		UInt128 k = 1UL;
 		var auto = new MersenneResidueAutomaton(exponent);
@@ -17,7 +17,7 @@ public class MersenneNumberOrderCpuTester(GpuKernelType kernelType)
             // TODO: When this lookup misses the snapshot, invoke the configured device to compute the
             // single required cycle on demand without persisting it so the CPU order path retains the
             // cycle-stepping speedups while honoring the no-extra-cache constraint for large divisors.
-			int allowMask2 = lastIsSeven ? CpuConstants.LastSevenMask10 : CpuConstants.LastOneMask10;
+			int allowMask2 = lastDigit == LastDigit.Seven ? CpuConstants.LastSevenMask10 : CpuConstants.LastOneMask10;
 			bool shouldCheck = ((allowMask2 >> (int)auto.Mod10R) & 1) != 0;
 			if (shouldCheck)
 			{

PerfectNumbers.Core/Cpu/MersenneNumberResidueCpuTester.cs

@@ -5,7 +5,7 @@ public class MersenneNumberResidueCpuTester
 	private ModResidueTracker? _mersenneResidueTracker;
 
 	// CPU residue variant using tracker + unrolled residue updates (no method calls in the loop).
-	public void Scan(ulong exponent, UInt128 twoP, bool lastIsSeven, UInt128 maxK, ref bool isPrime)
+	public void Scan(ulong exponent, UInt128 twoP, LastDigit lastDigit, UInt128 maxK, ref bool isPrime)
 	{
 		// Initialize/update Mersenne residue tracker and start a merge walk for ascending q divisors
 
@@ -29,9 +29,9 @@ public class MersenneNumberResidueCpuTester
                 step10 = step10.Mod10();
 
 		// Allowed last-digit sets for q depending on last digit of M_p
-		// lastIsSeven == true  => allow {7,9}
-		// lastIsSeven == false => allow {1,3,7,9}
-		int allowMask = lastIsSeven ? CpuConstants.LastSevenMask10 : CpuConstants.LastOneMask10;
+		// LastDigit.Seven => allow {7,9}
+		// otherwise       => allow {1,3,7,9}
+		int allowMask = lastDigit == LastDigit.Seven ? CpuConstants.LastSevenMask10 : CpuConstants.LastOneMask10;
 
 		// Predeclare temps to avoid redeclaration overhead in the loop
 		UInt128 remaining;