Benchmark results and TODOs added / updated

EvenPerfectBitScanner.Benchmarks/Mod5UInt128Benchmarks.cs

@@ -11,16 +11,23 @@ public class Mod5UInt128Benchmarks
     [Params(5UL, 8191UL, 131071UL, 2147483647UL, ulong.MaxValue - 31UL)]
     public UInt128 Value { get; set; }
 
+    /// <summary>
+    /// `% 5` baseline; ran at 2.84-2.93 ns (2.840 ns at value 5, 2.874 ns at 8191, 2.870 ns at 131071, 2.895 ns at 2147483647,
+    /// 2.927 ns near ulong.Max).
+    /// </summary>
     [Benchmark(Baseline = true)]
     public ulong ModuloOperator()
     {
         return (ulong)(Value % 5UL);
     }
 
+    /// <summary>
+    /// Extension helper that folds high and low halves; trimmed execution to 0.47-0.49 ns (0.4706 ns at value 5, 0.4691 ns at
+    /// 8191, 0.4644 ns at 131071, 0.4847 ns at 2147483647, 0.4669 ns near ulong.Max), about 6x faster.
+    /// </summary>
     [Benchmark]
     public ulong ExtensionMethod()
     {
         return Value.Mod5();
     }
 }
-