Add TODOs for divisor metadata caching

PerfectNumbers.Core/ExponentRemainderStepper.cs

@@ -44,6 +44,9 @@ internal struct ExponentRemainderStepper
 
         if (!_hasState || exponent <= _previousExponent)
         {
+            // TODO: Route these state resets through the ProcessEightBitWindows helper once the scalar
+            // Pow2MontgomeryMod implementation adopts it so delta stepping inherits the benchmarked
+            // 2× gains for large exponents.
             _currentMontgomery = exponent.Pow2MontgomeryModMontgomery(_divisor);
             _previousExponent = exponent;
             _hasState = true;
@@ -51,6 +54,8 @@ internal struct ExponentRemainderStepper
         }
 
         ulong delta = exponent - _previousExponent;
+        // TODO: Replace this per-delta powmod with the upcoming windowed ladder so incremental
+        // updates stop paying the single-bit cost highlighted in GpuPow2ModBenchmarks.
         ulong multiplier = delta.Pow2MontgomeryModMontgomery(_divisor);
         _currentMontgomery = _currentMontgomery.MontgomeryMultiply(multiplier, _modulus, _nPrime);
         _previousExponent = exponent;
@@ -67,6 +72,8 @@ internal struct ExponentRemainderStepper
 
         if (!_hasState || exponent <= _previousExponent)
         {
+            // TODO: Switch this reload to the shared windowed pow2 helper once available so CPU
+            // residue checks align with the optimized ProcessEightBitWindows timings.
             _currentMontgomery = exponent.Pow2MontgomeryModMontgomery(_divisor);
             _previousExponent = exponent;
             _hasState = true;
@@ -74,6 +81,8 @@ internal struct ExponentRemainderStepper
         }
 
         ulong delta = exponent - _previousExponent;
+        // TODO: Use the windowed delta pow2 helper here as well to avoid the single-bit ladder that
+        // currently lags behind the benchmarked implementation for huge divisor cycles.
         ulong multiplier = delta.Pow2MontgomeryModMontgomery(_divisor);
         _currentMontgomery = _currentMontgomery.MontgomeryMultiply(multiplier, _modulus, _nPrime);
         _previousExponent = exponent;
@@ -107,6 +116,8 @@ internal struct ExponentRemainderStepper
 
         if (!_hasState || exponent <= _previousExponent)
         {
+            // TODO: Replace this fallback path with the upcoming ProcessEightBitWindows helper so
+            // fresh Montgomery states also benefit from the faster pow2 ladder measured on CPUs.
             _currentMontgomery = exponent.Pow2MontgomeryModMontgomery(_divisor);
             _previousExponent = exponent;
             _hasState = true;