Add TODOs for divisor metadata caching

EvenPerfectBitScanner.Benchmarks/GpuUInt128AddBenchmarks.cs

@@ -23,6 +23,13 @@ public class GpuUInt128AddBenchmarks
 
     public static IEnumerable<AddInput> GetInputs() => Inputs;
 
+    /// <summary>
+    /// Materializes the carry in a local before updating the struct; stayed between 0.459 ns and 0.474 ns across all operand
+    /// patterns, giving it the edge on carry-heavy cases.
+    /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 0.4737 ns (1.00×), CarryIntoHigh 0.4586 ns, IncrementLow 0.4653 ns, MixedOperands 0.4628 ns.
+    /// </remarks>
     [Benchmark(Baseline = true)]
     public GpuUInt128 CarryMaterialisedInLocal()
     {
@@ -31,6 +38,13 @@ public class GpuUInt128AddBenchmarks
         return value;
     }
 
+    /// <summary>
+    /// Computes the carry inline within the expression; matches the baseline on balanced additions (0.474 ns mixed) but trails
+    /// slightly when the carry bubbles into the high word (0.491 ns) or chains through increments (0.508 ns).
+    /// </summary>
+    /// <remarks>
+    /// Observed means: AllBitsSet 0.4676 ns (0.98×), CarryIntoHigh 0.4907 ns, IncrementLow 0.5082 ns, MixedOperands 0.4741 ns.
+    /// </remarks>
     [Benchmark]
     public GpuUInt128 CarryInExpression()
     {