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/solution/Buyers/PriceList.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class PriceList
+    {
+        // placeholder implementation - real version would call the pricing service
+        private static readonly Dictionary<IceCream, int> BasePrices =
+            new()
+            {
+                { IceCream.Strawberry, 3 },
+                { IceCream.Vanilla, 2 },
+                { IceCream.Chocolate, 4 },
+            };
+
+        public int PriceFor(IceCream flavour)
+        {
+            return BasePrices[flavour] + SurgePricing();
+        }
+
+        private int SurgePricing()
+        {
+            return new Random().Next(3);
+        }
+    }
+}