Clarify residue stepper usage and update TODO tracker

PerfectNumbers.Core/Gpu/MersenneNumberResidueGpuTester.cs

@@ -26,7 +26,7 @@ public class MersenneNumberResidueGpuTester(bool useGpuOrder)
         UInt128 kStart = 1UL;
         UInt128 limit = maxK + UInt128.One;
         byte last = lastIsSeven ? (byte)1 : (byte)0;
-        var kernel = gpuLease.Pow2ModKernel; // Pow2ModKernel already executes the windowed pow2 ladder from GpuPow2ModBenchmarks.
+        var kernel = gpuLease.Pow2ModKernel;
         twoP.Mod10_8_5_3(out ulong step10, out ulong step8, out ulong step5, out ulong step3);
         step10 = step10.Mod10();
 

PerfectNumbers.Core/MersenneResidueStepper.cs

@@ -18,6 +18,8 @@ public class MersenneResidueStepper
         p = p0;
 
         // Inputs are already reduced in production; keep the old modulo guard commented for debugging reference.
+        // TODO: Confirm whether GPU execution ever constructs this stepper; if so, split the CPU/GPU initialization so
+        // the accelerator path retains the modulo reductions that prevent divergence.
         // M_mod = M_mod_m_at_p0 % m;
         // pow2_mod = (M_mod + 1) % m;
         M_mod = M_mod_m_at_p0;