Reapply modulo helpers and inline fast remainders

PerfectNumbers.Core.Tests/UInt128Extensions/UInt128ExtensionsTests.cs

@@ -201,6 +201,14 @@ public class UInt128ExtensionsTests
         value.Mod3().Should().Be((ulong)(value % 3));
     }
 
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void Mod6_matches_operator()
+    {
+        UInt128 value = (UInt128.One << 100) + 9876543210UL;
+        value.Mod6().Should().Be((ulong)(value % 6));
+    }
+
     [Fact]
     [Trait("Category", "Fast")]
     public void Mod5_matches_operator()