Windowed Pow2Mod implemented and integrated across repository.

PerfectNumbers.Core/UInt128Numbers.cs

@@ -6,6 +6,9 @@ namespace PerfectNumbers.Core
         public static readonly UInt128 Four = 4UL;
         public static readonly UInt128 SixtyFour = 64UL;
         public static readonly UInt128 OneHundredTwentySeven = 127UL;
+        public static readonly UInt128 OneShiftedLeft64 = UInt128.One << 64;
+        public static readonly UInt128 OneShiftedLeft64x2 = Two * OneShiftedLeft64;
+
         // TODO: Populate this table from the UInt128 residue benchmarks so GPU kernels can pull precomputed constants
         // without re-deriving them at runtime.
         // TODO: Extend the table with cached Mod3/Mod5 folding constants so both CPU and GPU residue helpers can reuse