Slice with lambda: add C# exercise projects and export

- Add SliceWithLambda.slnx + csproj scaffolding for ideas/slice-with-lambda/
  exercise/csharp (examples project + xUnit solution/test project)
- Add exercises/slice-with-lambda (C# version of the prefactored-longrange
  exercise with Buyers and SliceWithLambda projects)
- Add export/slice-with-lambda (nested git repo with exercise export)
- Ignore bin/ and obj/ build output at repo root

exercises/slice-with-lambda/tests/SliceWithLambdaTests/SliceWithLambdaTests.cs

@@ -0,0 +1,82 @@
+// Tests for the three Slice-with-Lambda solutions — step 4: the lambda as a stub.
+//
+// Condensed mechanics: https://sammancoaching.org/refactorings/slice.html
+// (where that page says mock, we use a lambda instead).
+
+using System;
+using Xunit;
+
+public class SliceWithLambdaTests
+{
+    // --- Example 1: metaphor-free (Calculator) ---
+
+    [Fact]
+    public void Guards_against_negative_input()
+    {
+        var calc = new Calculator(new SomeDependency());  // the real, flaky one...
+        Assert.Equal(0, calc.Calculate(-1, a => 42));     // ...never asked: stub not invoked
+    }
+
+    [Fact]
+    public void Throws_when_precondition_not_satisfied()
+    {
+        var calc = new Calculator(new SomeDependency());
+        Assert.Throws<InvalidOperationException>(
+            () => calc.Calculate(7, a => throw new InvalidOperationException()));
+    }
+
+    [Fact]
+    public void Combines_input_and_the_stubbed_value()
+    {
+        var calc = new Calculator(new SomeDependency());
+        Assert.Equal(84, calc.Calculate(0, a => 42));     // stub through the seam
+    }
+
+    // --- Example 2: Weather Station (SprinklerController) ---
+
+    [Fact]
+    public void Skips_watering_when_already_watered()
+    {
+        var controller = new SprinklerController(() => 200);  // pure lambda: no sensor
+        controller.ScheduleWatering();                        // water once...
+        Assert.Equal(WateringDuration.None, controller.ScheduleWatering());
+    }
+
+    [Fact]
+    public void Waters_longer_in_full_sun()
+    {
+        var controller = new SprinklerController(() => 200);  // sunny: > 128
+        Assert.Equal(WateringDuration.Medium, controller.ScheduleWatering());
+    }
+
+    [Fact]
+    public void Waters_less_in_the_shade()
+    {
+        var controller = new SprinklerController(() => 10);   // shady: ≤ 128
+        Assert.Equal(WateringDuration.Short, controller.ScheduleWatering());
+    }
+
+    // --- Example 3: Ice Cream Shop (IceCreamShop) ---
+
+    [Fact]
+    public void Skips_thawing_when_already_thawed()
+    {
+        var shop = new IceCreamShop(() => 30);        // pure lambda: no freezer
+        shop.ThawStock();                             // thaw once...
+        Assert.Equal(ThawAmount.None, shop.ThawStock());
+    }
+
+    [Fact]
+    public void Thaws_more_when_warm()
+    {
+        var shop = new IceCreamShop(() => 30);        // warm: > 25
+        Assert.Equal(ThawAmount.Medium, shop.ThawStock());
+    }
+
+    [Fact]
+    public void Thaws_less_when_cold()
+    {
+        var shop = new IceCreamShop(() => 5);         // cold: ≤ 25
+        Assert.Equal(ThawAmount.Low, shop.ThawStock());
+    }
+}