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/Buyer.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class Buyer
+    {
+        public Buyer(string name, IReadOnlyList<IceCream> favourites)
+        {
+            Name = name;
+            Favourites = favourites;
+        }
+
+        public string Name { get; }
+        public IReadOnlyList<IceCream> Favourites { get; }
+
+        public int ScoopsOf(IceCream flavour)
+        {
+            return 2;
+        }
+    }
+}

exercises/slice-with-lambda/solution/Buyers/Buyers.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    /// <summary>
+    /// Reference end state of exercise 3 (extract lambda, slide, extract method
+    /// with a function parameter — the pure rep of Slice with Lambda).
+    /// </summary>
+    public class Buyers
+    {
+        private readonly PriceList priceList = new();
+        private readonly List<Buyer> buyers = new()
+        {
+            new("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla }),
+            new("Bram", new List<IceCream> { IceCream.Chocolate }),
+            new("Celine", new List<IceCream> { IceCream.Vanilla, IceCream.Chocolate }),
+        };
+
+        /// <summary>
+        /// No-arg overload: callers unchanged, the "real" pricing function baked in.
+        /// </summary>
+        public int ShoppingListTotal(string buyerName)
+        {
+            return ShoppingListTotal(ByName(buyerName), priceList.PriceFor);
+        }
+
+        /// <summary>
+        /// The seam: the pricing source is injected as a function.
+        /// Testable: pass any pure function instead of calling the pricing service.
+        /// </summary>
+        public int ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)
+        {
+            var total = 0;
+            foreach (var flavour in buyer.Favourites)
+            {
+                total += buyer.ScoopsOf(flavour) * priceFor(flavour);
+            }
+            return total;
+        }
+
+        private Buyer ByName(string name)
+        {
+            return buyers.Find(b => b.Name == name)
+                ?? throw new InvalidOperationException($"Unknown buyer {name}.");
+        }
+    }
+}

exercises/slice-with-lambda/solution/Buyers/Buyers.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

exercises/slice-with-lambda/solution/Buyers/IceCream.cs

@@ -0,0 +1,9 @@
+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

exercises/slice-with-lambda/solution/Buyers/PriceList.cs

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+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);
+        }
+    }
+}