Add exercise 3 (Buyers, C#): the pure extract-lambda rep

New in-theme classes: Buyers computes a shopping-list total, PriceList
stands for the pricing service (random surge, flaky test). One loop, one
lambda-able expression — the third example teaches the move with nothing
else in the way. Pristine repo + reference solution (extract lambda,
slide, extract method with Func<IceCream,int> parameter, pure-lambda
test) + storyboard in the established style. Mirrors the kata's C#
setup (net7.0, MSTest); needs a build check on a machine with the SDK.

ideas/split-loop-with-lambda/exercise3/csharp/src/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;
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/Buyers.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    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 }),
+        };
+
+        public int ShoppingListTotal(string buyerName)
+        {
+            var buyer = ByName(buyerName);
+            var total = 0;
+            foreach (var flavour in buyer.Favourites)
+            {
+                total += buyer.ScoopsOf(flavour) * priceList.PriceFor(flavour);
+            }
+            return total;
+        }
+
+        private Buyer ByName(string name)
+        {
+            return buyers.Find(b => b.Name == name)
+                ?? throw new InvalidOperationException($"Unknown buyer {name}.");
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/Buyers.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ideas/split-loop-with-lambda/exercise3/csharp/src/IceCream.cs

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+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/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);
+        }
+    }
+}