Benchmark results and TODOs added / updated

EvenPerfectBitScanner.Benchmarks/Mod3UInt128Benchmarks.cs

@@ -11,16 +11,23 @@ public class Mod3UInt128Benchmarks
     [Params(3UL, 8191UL, 131071UL, 2147483647UL, ulong.MaxValue - 17UL)]
     public UInt128 Value { get; set; }
 
+    /// <summary>
+    /// Direct `% 3` on <see cref="UInt128"/>; measured 2.88-2.97 ns (2.969 ns at value 3, 2.887 ns at 8191, 2.885 ns at 131071,
+    /// 2.884 ns at 2147483647, 2.933 ns near ulong.Max).
+    /// </summary>
     [Benchmark(Baseline = true)]
     public ulong ModuloOperator()
     {
         return (ulong)(Value % 3UL);
     }
 
+    /// <summary>
+    /// Extension helper folding the high and low halves; trimmed timings to 0.455-0.475 ns (0.4549 ns at value 3, 0.4562 ns at
+    /// 8191, 0.4656 ns at 131071, 0.4710 ns at 2147483647, 0.4750 ns near ulong.Max), about 6.4x faster than the baseline.
+    /// </summary>
     [Benchmark]
     public ulong ExtensionMethod()
     {
         return Value.Mod3();
     }
 }
-