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/IceCreamShopSolution.cs

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+// Example 3 solution — Ice cream shop demo of Slice with Lambda (C#), after
+//
+// Lambda extracted (step 1), slid to the top (step 2), method extracted
+// below the guard (step 3). The constructor now takes the lambda directly,
+// so tests construct with new IceCreamShop(() => 30).
+//
+// Condensed mechanics: https://sammancoaching.org/refactorings/slice.html
+// (where that page says mock, we use a lambda instead).
+
+using System;
+
+public enum Warmth
+{
+    Cold,
+    Warm
+}
+
+public enum ThawAmount
+{
+    None,
+    Medium,
+    Low
+}
+
+public class Thermometer
+{
+    private readonly Random _rng = new Random();    // stand-in for the hardware
+
+    public byte Read()
+    {
+        return (byte)_rng.Next(50);                 // raw sensor value
+    }
+}
+
+public class IceCreamShop
+{
+    private readonly Func<byte> _readTemperature;   // the thermometer, as a lambda
+    private bool _thawedToday;                      // event-style client state
+
+    public IceCreamShop(Func<byte> readTemperature)
+    {
+        _readTemperature = readTemperature;
+    }
+
+    public ThawAmount ThawStock()
+    {
+        Func<byte> readTemperature = () => _readTemperature();  // lambda at the top
+        return ThawStockTail(readTemperature);      // pass the lambda on
+    }
+
+    private ThawAmount ThawStockTail(Func<byte> readTemperature)
+    {
+        if (_thawedToday)                           // guard: client state, stays
+        {
+            return ThawAmount.None;
+        }
+
+        _thawedToday = true;                        // set only after thawing
+        byte temperature = readTemperature();       // middle: the stubbed lambda
+        Warmth warmth = ToWarmth(temperature);
+        if (warmth == Warmth.Warm)                  // tail: complex behaviour below
+        {                                           // the extraction point
+            return ThawAmount.Medium;
+        }
+        return ThawAmount.Low;
+    }
+
+    private static Warmth ToWarmth(byte temperature)
+    {
+        return temperature > 25 ? Warmth.Warm : Warmth.Cold;
+    }
+}