Various micro- and medium- optimizations

PerfectNumbers.Core/UInt128Numbers.cs

@@ -1,3 +1,5 @@
+using PerfectNumbers.Core.Gpu;
+
 namespace PerfectNumbers.Core
 {
     public static class UInt128Numbers
@@ -9,6 +11,8 @@ namespace PerfectNumbers.Core
         public static readonly UInt128 OneShiftedLeft64 = UInt128.One << 64;
         public static readonly UInt128 OneShiftedLeft64x2 = Two * OneShiftedLeft64;
 
+        public static readonly GpuUInt128 TwoGpu = new(2UL);
+
         // 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