Add solution (after) files for the three C# demos: seam overloads, extracted tails, pure xUnit stub tests

ideas/split-loop-with-lambda/exercise/csharp/solution/MetaphorFreeSolution.cs

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+// Example 1 solution — Metaphor-free demo of Slice with Lambda (C#), after
+//
+// Lambda extracted (step 1), slid to the top (step 2), method extracted
+// below the guard (step 3), overload seam added (step 3b), b inlined
+// (final cleanup).
+//
+// Condensed mechanics: https://sammancoaching.org/refactorings/slice.html
+// (where that page says mock, we use a lambda instead).
+
+using System;
+
+public class Calculator
+{
+    private readonly SomeDependency _someDependency;
+
+    public Calculator(SomeDependency someDependency)
+    {
+        _someDependency = someDependency;
+    }
+
+    public int Calculate(int a)                          // overload: callers unchanged
+    {
+        return Calculate(a, a =>
+            _someDependency.mightThrowWhenPreconditionNotSatisfied(a));
+    }                                                    //   the real λ, baked into the head
+
+    public int Calculate(int a, Func<int, int> readB)    // the seam: λ is a parameter
+    {
+        if (a < 0) return 0;                             // guard clause: stays
+        return CalculateTail(a, readB);
+    }
+
+    private int CalculateTail(int a, Func<int, int> readB)
+    {
+        int c = a + readB(a);                            // b inlined
+        return c * 2;
+    }
+}
+
+public class SomeDependency
+{
+    private readonly Random _rng = new Random();    // stand-in for unpredictable computation
+
+    public int mightThrowWhenPreconditionNotSatisfied(int a)
+    {
+        if (a > 99)
+        {
+            throw new InvalidOperationException("precondition not satisfied");
+        }
+        return _rng.Next(100);
+    }
+}